the original trinity
The original trinity of gods in hindu mythology is varuna, mitra and aryaman, three aspects of sun, the first creating, the second maintaining and third destroying. This is said to originate with solar cult creating with its warmth, preserving with his light and destroying with his burning rays. They were sometimes though of as three separate gods and sometimes, the three aspects of the same.
this was succeeded by another trinity viz. indra, agni and vayu again representing three importsnt natural forces. aditya was considered to be one form of agni.
over time they gave way to another trinity, brahma, vishnu, mahesh performing the same functions as aryaman and thought of as one or different from each other according to the context. Only vishnu as preserver could have avtars.