I've turned my speculation in one thread about the Juna (of Spelljammer), Tiraphegs (from the 1st edition fiend folio) & Synads (from complete psionic), into something a little more concrete.
The Ancient Juna
In ancient times at least one civilization took the concept of the ‘Rule-of-Three’ to be more than a mere idea. They were a civilization known as the Juna, one of the elder races that ruled much of the Material Plane for a time.
Scereus Palatino (NG male human, wizard[diviner] 10/lorekeeper 3) of the Seekers states, “The Juna were in significance much before the period known as the classical age of spelljamming. They’ve predated the Mercane, K’r’r’r and many of the races currently now inhabiting much of wildspace. We know very little about them, except that they are roughly human-sized and had an unusual type of physiology that can be described as Tri-Symmetrical.”
“What we’ve learned about them has mainly come from a variety of ruins and hulks they’ve left for us out there in wildspace. There’s so much more we need to learn about them. However our research indicates that the legendary ‘ghost’ ship and demiplane according to some explorers, known as The Spelljammer appeared around the time of their civilization, thus leading many of us to theorize that it was a creation of the Juna themselves”
The Juna even made a few forays into the planes, as scattered ruins of their civilization can occasionally be found on the planes. Phrandahar De’kaati (LN male élan, psion[nomad] 14) a renowned member of the Planewalker’s Guild had this to say about the subject, “The Juna got themselves out here on the planes. I’ve seen some of their ruins out there. When you think about it they were like any other civilization, they had their share of powers they worshipped. Though now their realms remain unreachable to all. It’s not that their necessarily all in the dead-book, it’s just that the concepts are so barmy and inconceivable to most bashers out there, they couldn’t reach such places even if they went looking for them in their mindsets.”
“Many of the ruins of the Juna lie in the Transitive planes, floating through the Astral, Deep Ethereal and most of all deep within sea of blackness of the Shadow Plane. You see many of the psionic techniques of plane travel were actually designed by them. I’ve seen ruins of the Juna in silvery void that depict linked travellers doing the Astral Caravan. On the walls you’ll they even set themselves a bit of psionic circuitry that builds on the principles of such powers. I can tell you that the Juna were in the Astral Plane even before those Githyanki berks got there.”
It’s unknown how they related to other races at the time, but many think that the Juna were in conflict with the Aboleths, Ethergaunts, Beholders and Illithids which might have had empires at the same time as their civilization. Not much in the Juna ruins though indicates that they had much conflict with these races.
Though Hr’uibu’ken (CE Kaorti, wizard [transmuter] 5/ alienist 4) of the Kaorti has this to say about the subject, “The great beings beyond tell me something!!! Destroy the tripartite ones!!! Destroy all knowledge of them and their blasphemous past, and then the voices of the ones from beyond will reduce their screams down to whispers!!”
And then somehow the ancient Juna just disappeared, gone before most of the other races today began to explore space. No one really knows what happened to them, though there have been many unconfirmed claims towards encountering the Juna.
The Absence & the Tiraphegs
With the Juna gone, other races began to flourish across the Material Plane. It’s now theorized that where the Juna retreated to was another part of the Material Plane, separated from the space that many other planets existed in. This world or worlds that they retreated to could only be accessed through the expanses of the Shadow Plane. Such that no one could hope to find them and have any hope of returning to tell about it. It’s thought that the Juna actually had castes or subraces (most likely 3), which would explain some uncharacteristically different behaviour in their actions. One of these castes is thought to be the Tiraphegs.
A Tirapheg is a really odd and alien looking creature; they have three legs, three heads and three arms. Two heads have absolutely no features, except for the central head which has eyes and a nose. Two of their arms end in sharp points, while the central one has three fingers, all their arms is really flexible and prehensile like tentacles. On their stomachs a Tirapheg has what resembles a mouth with two short tentacles on either side. They’re described as having the ability to project multiple images of them, with blinding flashes of lights.
Encounters with Tiraphegs have been scarce but at least one adventurer known as Buktu Yarghik (CN male bugbear, barbarian 
had this to say about the subject, “Yeah I saw one to those terror-pigs or whatever you call them. It was after a night of heavy drinking, and I ran into one of those things, three legs and arms and all. I was shocked, so I looked at the thing all funny, and it looked at me funny. So I stared at it again, and it did the same.”
“So I laughed at it for about minute, and it just stood there and stared at me. Then I decided to tap it over the head with my greatsword to see if it would react. But the damn thing disappeared in a blinding flash of light. I tried to tell me drinking buddies about it, but they wouldn’t believe me. So then I told a bunch of scholars who were writing a book called the Obscure Folio and Catalogue of the Fiendish and Bizarre, and they paid me a bunch of money, that’s good since I can’t really read anyways.”
A couple of planewalkers once mentioned they came across a demiplane entirely populated by Tiraphegs and creatures known as Flumphs locked in mortal combat, however this claim has been described as, ‘pure screed’ by various more established and reliable Sigilian planewalkers.
Some theorize that the Tiraphegs are the lowest caste of the Juna (possibly one of the lowest), were encountered across the planes as they were part of a Juna expeditionary force meant to explore and observe the worlds that they retreated from.
The Arrival of the Synads
From across the Shadow Plane in more recent times, a new race has started to appear on a number of worlds. These people call themselves the Synad; they look and appear mostly human. Their fingers may be a little longer than humans, and they tend to have darker eyes, and are generally hairless, but for the most part Synads can pass for humans. That is unless they are viewed in the dim light when their reserve of mental energy is expended, where one can see that they have two ghostly and featureless heads bracketing the Synad’s head. This has lead a few to theorize that the Synads may be linked to the Tiraphegs somehow.
Synads typically don’t like to talk about their origins or even have their true nature known to others. But of what little they’ve shared, is possibly the most anyone will know about the Juna. Synads claim that their world is inhabited giant monstrosities of the mind which roam unhindered. Only one Synad known as Karala El Mienaro (N, female synad, Psion [Erudite] 16) has mentioned any link that their race has had to the Juna.
“The Juna still exist out there, it’s just that they’ve been hiding. Some of them have been collecting creatures from across the planes and vast expanses of space, to study them. I’ve heard of some use the term Zookeepers to describe these Juna. You see the truth is that we of the Synad happen to be Juna ourselves; I existed once as a member of the Tirapheg caste. The form of the Tirapheg wasn’t well-received across the planes, so we took on forms that would allow us to blend among the most common civilizations across the worlds.”
“The other Juna have long ruled above us, that when given choice of existing back in the hidden world, or out here in these worlds, we choose instead to migrate to these worlds. They still maintain their influence over us, as we are all Juna. You see every one of the Synad is of three minds. Our individual identities are of what we call the Overmind, it’s the core of our personality and souls.”
“The other two minds are of others. The mind we call the Collective is the linked mind of all Synads, and of other Juna. From the Collective the Juna know what is happening here, and the knowledge of all Synads can be shared. The last of the three minds is the Oracle. The Oracle is directly linked back to the Juna of the world we came from, is from the Oracle that the Juna can speak to us.”
“There are so many things the Juna want to learn about the worlds they left so long ago. Right now they seek to observe, waiting patiently to make their decision on what to do. But should they decide to return to the worlds they left; then I fear a great conflict will be ahead. They know that there are those out there who would stand in opposition to them, and thus the Juna would seek to ruthlessly destroy all of them. You should fear them as I do, as the Juna don’t have much regard for the rest of you either.”
So far no one else has confirmed or denied anything said by Karala, but her with what’s she’s shared has so far sounded like the most plausible and likely explanation to this whole mystery. Shortly after sharing this information Karala, went into hiding. Many said it was the presence of an assassin known as Yurlai Bahash (NE female synad, Lurk 14/ Soulknife 6) who arrived once she learned of what Karala said.
Still the mystery of the Juna and their alleged descendants is something that’s baffled many of centuries, as there are still so many unanswered questions.