Yes, you can have 3 tangs in a 90.
Most people would freak saying it's mean It all depends on which ones. I would not put a Purple and a yellow together...mainly becasue thay are of the same genus.
Typically, if the body shape is the same, they will fight. Also, be sure to addd all 3 at the same time...less stress...less fighting at least in my experience.
HTH