I would love if ranked, masuri, arena and whatever else gets their own summon set
That said, in the mean time:
Slot 3 for ranked (same in all team line up)
Slot 1 for the summon you use on the team on the formation (unless it use the same one as ranked, in which case this slot is whatever)
Slot 2 for all the more "volatile" stuff like masuri, arena etc