Well at the point I am at in the game (beat it, back on Pulse) I can get away easily with Lightning, Fang and Hope as RAV, COM, RAV.

I also have COM, COM, RAV
MED, SAB, SYN
RAV, SEN, RAV
RAV, COM, MED
MED, SEN, MED

About 90% of the time I don't need to switch from RAV, COM, RAV when in a fight.
If it is small scale to medium scale enemies and if there arn't too many in the fight.
Other wise I will be switching back and forth a lot. Or I will summon an Eidolon.