This trick requires your dog to know SHAKE.
To teach this trick, start by putting a treat under one cup. Let your dog see the treat and sniff it. Put your hand on top of the cup and say SHAKE. As soon as your dog reaches her paw up, pull your hand away allowing your dog to knock over the cup. Give her loads of praise by saying GOOD CUP. Continue with this pattern and attention until she is able to find the treat with three cups.
Ultimately say, WHICH CUP? and your dog will look like a genius when she knocks over the cup with a treat.