Does this happen to be a vocal piece with words to sing? When there are three lines, normally the top one is something other than the piano portion. The bottom half should be the left and right hand of the piano part. If this is a vocal piece, the top line would most likely represent what the singer would sing. The middle line would be the right hand for the piano, and the bottom line would be the left hand.
I'm not sure if this is what you mean or not...