how do u use party members?

[Shayo]

is it possible to have three other party members to fight with you while on single player mode?

[ZellMog]

Nope.

[Garland]

At best, you can choose which of 8 characters you made to take out for a particular outing, but doing so is really counterproductive. Only the character you use gets the artifact at stage's end. I'd rather have one well levelled character than 8 weaker ones. Also, you only get one chance to reply to each letter. If you want to get the most out of your trade, you're best off playing the game with a single character to reply to letters and better your family relations. Besides, for all intents and purposes, the races play about the same. The difference in a stat between the weakest and strongest race in a particular category is no more than a single artifact's stat boost.