Anyone, feel free to correct/add to this. But typically when you have 3 staves one of your hands, usually the left hand will cross over the right hand to play the 3rd staff. Music is sometimes written in 3 staves so its easier to read. For example, the left hand will have bass clef as usual, the right hand has treble clef, and then a 3rd staff above that for the left hand. Most songs are written so that the left hand plays something in bass clef, then has enough time to cross over the right hand to play the 3rd staff. Ask me to clarify this if you don't understand or if I'm just plain confusing you. Rachmaninoff's prelude op.32 no. 2 goes into 4 staves, 2 for each hand.