I don't mean this to sound too harsh, but you should NEVER house snakes together from the SAME species, let alone different ones. You are talking about species that are drastically different sizes and need different requirements. Go get two 30-40 gal tubs for your ball pythons and put your boa in the big one you already have. You will regret it big time if you keep them together, your snakes will become sick, stressed, and possibly dead if the boa gets defensive. You will not find a single reputable source that tells you housing balls and boas together is ok.
Also, if your balls are male and female, you are going to eventually get eggs whether you want them or not. Are you prepared to care for them? Either way, separate them. If you want to breed them, research what needs to be done and put them together only to breed.