Triennial rhymes with:
biennial, bicentennial, biennial, centennial, decennial, millennial, perennial, quadrennial, bicentennial, biennial, centennial, ceremonial, colonial, company'll, decennial, intracranial, lineal, matrilineal, matrimonial, menial, millennial, patrilineal, perennial, quadrennial, testimonial
Triennial sounds like:
terminal, terminally, thermal, thornell, thornhill, thornley, tramel, tramell, tramiel, trammel, trammell, tremel, treml, tremmel, trunnell, turmel, turmoil, turnley