If the SO was interested in the idea of a threesome, I’d rather he float the idea and then we look for a third together. Or he could phrase it as, “Want to have a threesome? If yes, then X is willing.” If you’re in a committed relationship, sharing a fantasy with your SO should be the most important part.
I just realized what’s iffy about the way the request was phrased in the question. It means that the SO and the friend have discussed the possibility of a threesome beforehand. To me, that has two problems. For him to discuss your private sex life with friends is disrespectful. He also should have asked you about having a threesome first, and if the answer was yes, then asked the friend.
Going behind your back to ask the friend first, before he knew if you were interested, is the wrong way to go about it. It makes me think that he’s overly interested in having sex with this specific friend. Ideally, there should be a lot of trust between everyone before you have a threesome. His actions wouldn’t inspire a lot of trust in me.
But hypothetically, if the SO and I had talked this over beforehand and we were on the same page about it, and both attracted to the third, then yes.