There are so many ways this could go wrong and you could lose everything you have. If I were you and I loved this man and was quite happy the way things were, I'd not get involved with this. I wouldn't even try this until she could handle her husband having sex with me alone. Once we established a sexual relationship as well as the romantic one we already have, and we were all into it, I'd consider having some threesomes for fun. The key thing is that I'd be doing it because we could and not because we have to.
Personally, I would only consider a threesome if it was literally for a male partner that was really into it. I'm straight but I like to please and I'd do it as long as the other girl knew that was the only reason I was having sex with her.
Sex is very important to me, I need sex to feel love. But, if I did happen to fall in love with someone in the absence of a sexual relationship, someone in the position your partner is in, I could see myself considering this arrangement purely because I'd want to have sex with my partner, and this would be the only way to achieve that ethically.That does not mean that it would be right to have sex with someone under those sorts of conditions, and it would probably make me feel deeply unhappy and kind of... dirty too. Dirty is the wrong word. It would just make me feel like I had sex with someone for all the wrong reasons. Two people actually. And that would fuck with my mojo.