You can walk a path that trails all three resorts. Or you can use the boats at citywalk to get to them. I would take the boat to PBH and then walk back to HRH then take the boat to citywalk and boat over to RPH and back in order to save a little time.
I remember when we visited PartyofSix on our last trip ( HI Tammy! ) the captain of the boat pointed out that if anyone on the Portofino boat wanted to stop at the Hard Rock just let him know as you board. They will make the stop if you ask.