You could do a trio of multies with that foot print but you would have to watch your water parameters very very closely. Since they don't have a big bioload it shouldn't be a problem. Do watch to see if you have an outcast and remove it to another tank. Usually happens with extra males.
I would say depends on the species. The Multifaciatus are soooo tiny (just got 4 myself) that I think a trio would be just fine in that size tank. If you get some old enough to distinguish sex, get only 1 male and 2 females.