My SO wants a threesome. (Added Info)

My SO wants a threesome or a foursome. His excuse is to give me more pleasure. (Double the pleasure).

We've been together for three years. He says it can be something we'll only try once to see if we like it or not.

I don't know what to think. I need advice.

I'm not particularly the jealous type and I don't know if I have a preference between male or female. But this is the man I want to be with for the rest of my life and I am scared the wrong memories will stick with us.