also the bull head will depend on male or female I caught both at the same time both around 5inch the male reached 10 in 6months where as the female is only 7
with just 3 sunfish, odds are you'll just end up with one anyway. why not save on the deaths and just get one to start. if you can sex them and have only females you might be ok.
or go with 6 or 7 so the toughest one can't decide who to kill first.