The Roman Triads (so much different than the Chinese Triads)
A Triad is a group of three Gods, kind of the "primary" Gods worshiped in Roman culture. These Triads changed three times throughout Rome's history. First was the Archaic Triad, then the Capitoline Triad, and lastly the Plebeian or Aventine Triad. Triads were generally constructed as above; great statues placed within a great temple located at a central location so that all could come an pay homage. Some of these temples remain today and some do not.
The Capitoline Triad
Two distinct Capitoline Triads were worshipped at various times in Rome's history, both originating in ancient traditions predating the Roman Republic. The one most commonly referred to as the "Capitoline Triad" is the more recent of the two, consisting of Jupiter, Juno and Minerva. The earlier triad, sometimes referred to in modern scholarship as the Archaic Triad (more on this in a moment), consisted of Jupiter, Mars and Quirinus and was Indo-European in origin. Each triad held a central place in the public religion of Rome during its time.
Jupiter, Juno and Minerva were honored in temples known as Capitolia, which were built on hills and other prominent areas in many cities in Italy and its provinces, particularly during the Augustan and Julio-Claudian periods. Most had a triple cella (which is like a room within a room within a room and will hence forth be known as inception architecture).
Although the word Capitolium could be used to refer to any temple dedicated to the Capitoline Triad, it referred especially to the temple on the Capitoline Hill in Rome known as aedes Iovis Optimi Maximi Capitolini ("the temple of the Best, Greatest, Capitoline Jupiter") - which certainly seems modern in its "hype factor". The temple was built under the reign of Lucius Tarquinius Superbus, the last King of Rome prior to the establishment of the Roman Republic. Although the temple was shared by Jupiter, Juno and Minerva, each deity had a separate cella, with Juno Regina on the left, Minerva on the right, and Jupiter Optimus Maximus in the middle.
The Plebeian (or Aventine) Triad
We're going a bit out of order here, sorry. On the timeline of Roman history it went: Archaic, Capitoline, Aventine. I should probably rearrange this issue to reflect that order but full-disclosure: I'm half-way through and I'm already 20-minutes late to a family function. LOGICAL ORDER BE DAMNED!
The Aventine Triad is a modern term for the joint cult of the Roman deities Ceres, Liber and Libera. The cult was established ~493 BC within a sacred district on or near the Aventine Hill, traditionally associated with the Roman plebs. Later accounts describe the temple building and rites as "Greek" in style. Some modern historians describe the Aventine Triad as a plebeian parallel and self-conscious antithesis to the archaic Capitoline Triad of Jupiter, Mars and Quirinus and the later Capitoline Triad of Jupiter, Minerva and Juno. The Aventine Triad, temple and associated ludi (games and theatrical performances) served as a focus of plebeian identity, sometimes in opposition to Rome's original ruling elite, the patricians.
The Aventine Triad was established soon after the overthrow of the Roman monarchy and establishment of the Republic. Rome's majority of citizen commoners (plebs) were ruled by the patricians, a small number of powerful, landed aristocrats who asserted a traditional, exclusive right to Rome's highest religious, political and military offices. The plebs not only served in Rome's legions: they were the backbone of its economy – smallholders, labourers, skilled specialists, managers of landed estates, vintners, importers and exporters of grain and wine.
Against a background of famine in Rome, an imminent war against the Latins and a threatened plebeian secession, the dictator Aulus Postumius Albus Regillensis (what a name!) vowed a temple to the patron deities of the plebs, Ceres, Liber and Libera on or near the Aventine Hill. The famine ended and Rome's plebeian citizen-soldiery co-operated in the conquest of the Latins. In 493 BC, a new built temple on or near the Aventine hill was dedicated to the Triad and Rome's first recorded ludi scaenici (religious dramas) were held in honour of Liber, for the benefit of the Roman people. Liber's festival, the Liberalia, may date from this time.
The Archaic Triad
So archaic we don't have a photo of it, so here's a picture of something equally awesome!
This is a statue of the Roman god Dis Pater. God of land-based riches and, eventually, of the Underworld. He was then absorbed by Pluto and then Hades.
Now back to business!
The original three deities thus worshiped, now more commonly referred to as the Archaic Triad, were Jupiter, Mars and Quirinus. This structure was no longer clearly detectable in later times, and only traces of it could be identified from various literary sources and other testimonies.
Georg Wissowa, in his manual of the Roman religion, identified the structure as a triad on the grounds of the existence in Rome of the three flamines maiores, who carry out service to these three gods. He remarked that this triadic structure looks to be predominant in many sacred formulae which go back to the most ancient period and noted its pivotal role in determining the ordo sacerdotum, the hierarchy of dignity of Roman priests: rex sacrorum, flamen dialis, Flamen Martialis, flamen quirinalis and pontifex maximus in order of decreasing dignity and importance. He remarked that since such an order did no longer reflect the real influence and relationships of power among priests in the later times, it should have reflected a hierarchy of the earliest phase of Roman religion.
Wissowa identified the presence of such a triad also in the Umbrian ritual of Iguvium where only Iove, Marte and Vofionus are granted the epithet of Grabovius and the fact that in Rome the three flamines maiores are all involved in a peculiar way in the cult of goddess Fides.
Sorry, I had to include that.