I have this issue too. I have three partners too. Some I have sex with more than others, but I find my tank empty and I feel fulfilled all the time. Great for me, not so much for them. This has been going on for three+ years now.
I have talked to all of them and told them that if they don't get enough they will have to find it elsewhere. One is married and has that option, but the other is mono and the other still just has only me right now! Lots of work!
I do my best to keep really good boundaries with sex. I do my best not to feel bad and not to take on the responsibility of satisfying them all the time. I also have a responsibility to myself too. In fact, I come first. I spent a good two years not masturbating and forgetting my own body. Recently I decided that I should stay in touch and have actually found that I had neglected my connection with myself. So I have been re-discovering myself.
This means less sex for others though.... Thankfully I have a very high desire for sex and am happy with once a day. I find I don't ever have to go without
All my bases are covered.
I would suggest making sure these women know that this can't go on forever. It really can't. Please don't take viagra just for them.... there are other things that can be done and really, if they are horny and you are not up to it, they can lie next to you to feel close and satisfy themselves sometimes. Sex doesn't have to be swinging from chandeliers all the time, geesh.