Advice on a threesome

So the man I have been with for the last three years wants us to have a threesome with another man (his friend). Now mind you, this man is the only man I’ve ever had sex with. And I really don’t feel comfortable with his friend or even having the threesome all together. I don’t want two dicks, I just want my mans. But he doesn’t understand how it is for someone like me who’s only ever been with him ):

Can those that have done them give me advice? I have bad anxiety and need to know how it is.

EDIT: The thing is before we got into a relationship he considers me talking to another man (as a friend) cheating. I don’t believe I did anything wrong. But he thinks I did. He also has a big thing with threesomes! We have done it with another woman, which I was completely fine with. I just don’t want two men! But now he wants to watch me with another man so he can see how I look being fucked. And the only person he will do it with is the guy he’s done threesomes with before. And this man is old, like my father, old. Which is why I don’t feel comfortable with it. It makes me sick to my stomach. But I have to do this in order to make him happy and our relationship better.. although he said that when we had the threesome with the girl. And nothing changed. I feel its degrading for him to make me do something I don’t want to do, but I literally have no choice.

):