At least 70 gallons IMO. If you use the one inch per gallon rule you'd need a 50 gallon (16'' * 3 = 48). Sharks need a lot of room to swim so the more the better. So at the very least I'd say 70 g.
You've got 3 of them so thats a good start. They seem to be pretty hard fish, but I've never kept them before.