Cuck or Threesome???😳

My gf and I have been dating for about a year, we have great sex and we’ve never had any real issues in the physical or emotional department are we are quite similar personality wise. Recently the idea of spicing up our sex lives has come up and my gf was talking about how one of her friends (another girl her age) had mentioned jokingly about having a threesome and the idea has been floating around in her head since. We talked about it briefly and I said how I thought how it would be hot to see her with another girl. She seems to be on the edge about the threesome but then I mentioned just watching and interacting with her and not the other girl to which seemed to spark a little sexual curiosity in her. How do you guys think I should go about this? Try and talk her into the threesome and talk about how it’s a fun and sexy thing for us to do together for the both of us? Or should I stay the cuck for now, which I may enjoy as I feel secure in our relationship, but never done this before. Or is it just a bad idea?