before adding any more fish you should probably double your live rock. you really want about 1.5-2lbs per gallon, if not more, for best filtration.
after you get the rock, then add a fish, let the biofilter adjust, then grab the three cardinals if you want them.
most likely, they will breed btw. you'll most likely get at least one female and one male, but even if you get 3 males, i believe they switch genders if needed.