While the Trinity itself is probably not prehistoric, its origins clearly are, from pre-Christian and pre-civilization times, in other words, from hunter-gatherers. The oldest known triskelions are said to be the ones on the Neolithic monument Newgrange.
The meaning of the Trinity symbol is the following:
Father = source or cause or as chaos theory says "initial conditions"
Son = outcome or effect or the resulting event
Holy Spirit = the perpetual motion behind the whole world, or the constant change, or simply put: energy
These three things together are God aka Nature, or as I call it the entire infinite and eternal world.