Ever had a 3some

My SO has been talking about bringing another girl into the relationship sexually and just won't drop the subject...... He knows that I don't mind him "judging" other girls he sees because let's face it, they will do it no matter what, so there is no need in getting mad about it or telling them not to but lately he is saying "I'd do her" about some chick every 2 hours, at least, and it's getting old. I feel like he wants more than me and that I am not good enough for him.... Need some advice please....