Any expierienced Redbellied Piranha keepers ?

What would be a good tank size for 3 piranhas ? Is it ok to keep only 3 ? What are some good tank mates ? Diet ? I still dont have the tank but im going to have it like an amozonian look. Please only expierienced keepers ( they would know what to do or what not do). Aditional information is welcomed as much as possible . Thanks in advance.