Threesomes

How do you approach the subject with your boyfriend/SO that you've never been with a girl, but you want to try? Or at the very least, tell him you've always wondered what being with another girl would be like? I could never outright ask him for a threesome, I'd be too embarrassed and afraid he would disapprove. I'm not sure if he would think he's lacking or not satisfying me, and that's not the case. I am very pleased with him and all that he does, I just think it would be really fun and really hot to play with a girl! Any suggestions? Any advice? Any experienced ladies wanna tell me what it's like? Anything! Lol