Buffs, attacker, and healer on most cases for P3. This was partly because I went against the final boss with low level Personas(quite literally, most of mine were 10 or more levels below me, and my healer was Titania). Of course while playing through you have to consider elements during the floor boss fights. What they're weak against, they're strengths, etc.
Unless they have neither so just go with whatever works.
Unless they have neither so just go with whatever works.