I think the three Miis on their own slots would work a lot better for organization and unlocks. Smash 4 wasn't really clear that Miis were three required run through various modes, and high scores and stuff were only saved for one type. There's also unused Xander lines for each Mii, so they wouldn't even need his help.



And Miis are now unlockable (well, they kinda were in Smash 4, but not the same way), so you'll need to battle them. But would they make the fight 3v1?



There also will need to be a default Mii for each type, because Miis are not required on the Switch, and someone may not have any to pull from.



Also, like you said, scrolling through a dozen or so different Miis to find the one you want is annoying. Splitting them into three slots helps shrink down the menu's clutter.