You can cap the unused wire (I think). While you can daisy-chain the receptacles, you can't run them in series (they must be wired in parallel if you understand the distinction of series/parallel). Just use the box in the middle as a junction box and branch off to each of the others. Or just go to one, then around back to the other. There is a code issue about box fill, so the quantity of wires and cables in the box will mandate a minimum size of the box - you have to have enough room to bring the wires in, make the connections, and then mount the receptacle in place without it being too cramped.