I found this site today and thought it was another good mechanical alternative for cruise control. I couldn't find the R3 listed so I contacted them and got this response:
The BrakeAway P/N 5CPO2 will fit your Rocket lll. It is not on our fit list because the cruise control will take up about 3/4" of your grip space. For someone with large hands this will not leave enough room. Some people trim down the flange to give them a little more room.
I just put mine on from my VTX. I had them on Kury grips, but I seem to like the stockers on the Rocket. Works well. I did have to modify the brake lever piece, as it wouldn't actuate with the R3T's big levers.
That would have to be some pretty large hands and if I had hands that big I think I would put them to better use..............say a hand model??!!!
(crackin' myself up again)