If I were in your shoes, I'd probably sell the SU-800 and replace it with a used SB-800 and keep the PWs. With that, you'd have a flash that can:
Operate as an on-camera CLS/AWL commander of your two SB600's with all the functionality of your current SU800
Operate as a third remote (in one of two remote groups) commanded by your camera's pop-up flash.
Operate as a third remote in your system if you choose to get another FlexTT1
Operate as a third remote in SU-4 mode while the SB-600s are commanded by the PW system (the SB800 would then be triggered by the flash from one of the SB600s...you'd have to keep your PW system in manual mode for this to work so that any preflashes didn't trigger it prematurely).