I was just wondering is it better to train all three girls in every dress sphere or just allocate each sphere to one girl. Are there any combinations for this that are better than other?

I allocate each dressphere for one particular girl except for the following: Lady Luck, Mascot, Dark Knight, Trainer, and the individual "ultimate" dresspheres (i.e. Floral Fallal). This is because it can come in handy to have more than one of these in the party at the same time. But really, there really aren't any cases where you need 3 thieves, 3 songstresses, 3 white mages, etc. Even 3 berserkers is kind of overkill and possibly risky.

If I end up needing a particular dressphere, say, an Alchemist, I switch Rikku into alchemist mode b/c I've maxed out Rikku's alchemist abilities. Trust me, for the most part, it's much easier to just max out one of each and designate one person for them, whomever you feel it appropriate. Seriously, it would far too long to max out all the dresspheres for all 3 girls.