Katsuyu, Manda, Gamabunta || Legendary Summoning Characters

I've a question, so if Katsuyu isn't the topmost character, what happens then? If she is the topmost, then the other two slots are used up by her like the current Bijuu's ingame. But if she's not then what? The other chars remain as they are?
If that's the case then she really doesn't have too much going for her as a char, bcuz she's only able to heal herself and no one else on the team. Her damage is minimal and would need constant reds to do damage. Doesn't feel too good to use imo if she does everything for herself when she could be oriented to be a really good support for the team