I have heard that P's are generally more aggressive with each other and form a heirarchy over time, and the thing about fish with that sorta social structure is that in a tight space it could be a mission.Getting 2 filters won't be a problem. I'm new to piranha but not completely new to aquariums. I had 2 saltwater tanks a few years ago but had to get rid of them when I went to college. I also know red bellies aren't the vicious predators people like to think about when they hear piranha.
I have nothing against piranhas, but I couldn't imagine keeping 3 fish that end up on the larger scale at 8-10"+ (?) in a 75. The only thing is if things get ugly there is nowhere to run and water quality can run downhill. I'm not trying to bash anyone or anything I just think that there are much more logical choices for stocking in a tank this size piranha wise. Just my 2c