Robotics really need all three disciplines. I think it's great that you've already taken CS and EE classes!
To know exactly what's needed for the program you're thinking about, you probably should ask the program director and the professors at that program.
However, in general, I'd say that someone who has a degree in one of (ME, EE, CS) and has an understanding and hobby-level knowledge of the other two, should be able to do fine!