I'm a guy who has a desire to please. For some reason I'm wired to enjoy giving rather than receiving. I'd bet a psychologist would have a field day with me. I hated Christmas from a very young age because I don't like gifts. My parents used to give me gifts with the receipt, knowing I would return whatever they got me. And I always made sure they got the credit back. I've always loved giving pleasure to partners, with no need for reciprocation. I feel bad when a woman gives me a blowjob and I'm not able to reciprocate. Yes, it feels wonderful during, if I can get my mind right, but afterwards I feel selfish. I'm always willing to please a woman orally, digitally, and/or please with toys. When I hear friends talk about sex, it is always about their pleasure. They cum, woman, eh, maybe. I always found that when you give a woman an orgasm (or multiple), they're likely to return for more. And I'm always willing to provide it.
With that said, I would think that after three years of watching you with other men, knowing that you have no problem being sexual with another man, etc that even I would want another woman involved, whether it be a FMF or MFMF. I would think there comes a day when he realizes that you're, well, kinda selfish. It may not be soon, but I bet it'll happen.
I'd recommend not continuing with MFM threesomes until you're able to open your mind to him getting more than a random blowjob while you're getting ^$^%$&^ by other men.