What is inside that one labeled "R"? It has a plug on it. If it has side channels as well as a channel at the bottom, it could be a power beyond port ... not sure though. If it is, you would need to get a power beyond sleeve to use it. As I see it, it looks like your valve on has 6 lines working right now, and "R" could be the 7th.
Also not sure why there is so little flow to the 3pt? I wouldn't hurt to take it apart and check the "O" rings to see if everything is in good working order. Again, not sure about this since I am not familiar with that valve, but if "R" is a power beyond port, you could feed the 3pt from that port I think.