BRUNER247;4098389; said:3 in a 70gal isn't that just asking for aggression between em? Get as big of tank as you can find n afford for more fish!
I have 3 in an 85 gallon and I have no aggression amongst them. The worst thing that ever happens is one of them having a fin nipped at feeding time.
K9.. for you I'd say it just depends on what you want to take on. The more fish you have the more activity you will have.. but also need a bigger tank and all the responsibilities that go with it. Whatever you want out of the tank and the available space you have should tell you what size you'd want to have. Minimum for 3 piranhas would be like an 70-85gal.