In my search, I find it handy to reclassify these triads into flexible categories, recognizing that these religions were so prone to creative symbolism and lateral-thinking analogy. So I'd like to suggest some helpful groupings as follows:
The sun is divided into three parts: three seasons (winter, spring-summer, autumn) and three daytime periods (morning, noon, evening). Think "Father Time". This stems from the heliocentricity of Etruscan religion.Etruscan: Tinia Cilensal, Tinia Thufal, Tinia Thneth (Trinity of Tinia); Snake, lion, goat (Chimeric Triad).Seasonal triad:
Roman: Jupiter Summanus, Jupiter Fidius, Jupiter Tonans.
The year is divided into three seasons (winter, spring-summer, autumn) with focus on agriculture and cyclical climate changes. Think "Mother Nature".Etruscan: Maris Husiurnana, Maris Halna, Maris Isminthians (Trinity of Maris).
Roman: Jupiter, Mars and Quirinus (Archaic Triad); Ceres, Liber and Libera (Avertine Triad).
The three brightest bodies in the sky happen to be the sun, moon and Venus (in that order). Thus: Tinia the sun, Menarva the moon and Uni the planet Venus. This is a twist on the pre-existing Babylonian triad.Etruscan: Tinia, Uni, and Menarva (Capitoline Triad).Roman: Jupiter, Juno and Minerva (Capitoline Triad).