If you wont gonna add any other tangs then a group of three hippos are ok IMO
You cant put angels, puffers, trigger w/ sharks and rays because they can bite/nip them
Maybe a school of very small fish would be very cool, i mean a trio of hippo tangs (they will probably swim together) and a school of small red or orange fish like apagon sp.
I m sure aggressive forum will help better but i really like the cardinal and shark combo