Has anyone used raspberry pi 3 for mining?
Mine what ? Mining needs some equipment that has the ability to solve blocks rapidly.
That's why gpu cards are used because they have that.
You guessed it, you can't do anything considered as mining on a raspberry since it has no decent cpu and you can't attach gpus to it.You need to build a proper mining rig if you're interested and have the funds available.
However, I'm not sure but I think you can setup a masternode on it. Research more about masternodes and if it's profitable but it needs you to have some basic knowledge in linux.