Well, there are 14 instantly playable characters on the select screen. There are two bosses and two hiddens as well.

You can't unlock Motaro or Shao Kahn, but you can unlock Cybernetic Smoke and play against Noob Saibot (who is just a blacked-out Kano).

There are various ways to unlock them depending on what version of MK3 you are talkin about...