The third turn skills are for boss killing (and raid boss killing) or for when your squad power isn't high enough to smash the enemies before that. With what we currently have, it's probably best to just stack them together with high single target damage girls. For most of the current content I find the front-loaded damage from the turn 1 boost girls more useful though, since like you said most fights don't last 3 turns (pretty much just raid bosses and the final bosses of 12-5 and the some of event maps).