Please help identify cactus!!!!?

I just bought a cactus, but it didn't say what kind it was!!!! I've looked all over the internet, but I can't seem to find the name of it. It has a stem that is not sharp, and resembles a 3 sided prism with the sides squished in a little. On top is a large, round, red ball that does have spikes on it. It's very small, only about 3.5 inches tall. If anyone has any clue as to what kind of cactus this is, please let me no. If you know of a good website with pictures to help identify it, that would be much appreciated!!! THANKS!!