My girlfriend wants a threesome but I think it’s an excuse to cuck me [cuckold] [first time]

A little background is we’ve been together since we were 15 and are 21 now. She’s only ever had sex with one other person besides me and she’s the only girl I’ve been with. About a month ago she told me that she fantasized about having a threesome but only would want it with another guy. I was definitely intrigued so I started asking questions and seeing what she wanted out of it and why. She told me ideally it would be with a bigger guy with experience. She wants the experience to be focused on her and the other guy while I’m there mainly for support and to eat her out. I asked her about some scenarios she had in mind and what she said was – me sucking him off while I ride your face or while you eat my ass. She also said she had a fantasy about fucking him while I licked her ass at the same time. All of this sounds very hot to me and I want to be able to give her this great experience! Just don’t know where to start or go from here