In older RPGs, 4 characters or even 5 fighting in one fight was common. Ever since Chrono Trigger, it seems, RPG developers are religiously attached to 3 characters and no more.
I'm just thinking of traditional, turn-based RPGs (or pseudo-real time, like the ATB system). Final Fantasy, Phantasy Star, Lunar, etc. Not action RPG's (Diablo) or TBS's (Shining Force)
What's your favorite, and why?