Well, you COULD mine on a Raspberry Pi 3, but it would be HIGHLY ineffective and very slow.
If you get a GPU, like you said, then I think it may work (providing the Raspberry Pis CPU can handle running the mining algorithm- which it can). It would probably be a very effective mining rig because the Raspberry Pi does not use much power and it is quite cheap.
Good luck and tell me if you try it because i am interested in the results!
