Firsttime with a woman- threesome

Ive been seeing a guy who is in a open marriage. He has asked if id be interested in having a threesome with his wife (not thier first time) . Ive never been with a woman but I am curious, and I think they are both attractive. They have offered to made dinner and we see how it goes, but i am still nervous. Did anyone "research" it beforeband, or can offer tips. Side note : I'm not thier native speaker, so worried i might miss something if the suggestion is just, "go with it"