A three-member team allows you to send the game's mandatory love-triangle into combat without unnecessary extras, which is certainly an advantage in VII and VIII.

Four people is good from a tactical point of view, but it's less realistic, since the 'formation' is always spread out quite wide, and characters often stand around idle for long periods while the other three do their stuff.

FFIV's five-strong team was nice because it allowed virtually the entire core cast to fight together.

Meh, usually whatever is chosen for a particular game is what works. FFVI is just dandy with a party of four, X-2's battles are fast and frenetic with a party of three.