Threesome??

So my partner wants to have a threesome for the first time. We been together 10 years. I’m 50% yes 50% no. For one I don’t have any body in mind for our third person. He wants it to be his ex (because she been willing since he stop talking to her 10 years ago) that he was with right before we got together. I don’t like that thought but then again I’m a very jealous person. I don’t want to ask my friends and be judged. What do you think about his ex? Is a threesome really that common?