Threesome

My girlfriend wants to have a threesome I could tell when I was jokingly talking about it, and she already had 3 girls in mind that she said she would have sex with. It made me mad because I feel like threesomes mean whatever I’m doing isn’t good enough and I just get jealous. Also I feel like it’s kind of degrading. What should I do? Because I want her happy with our sex life, but I don’t know how else to spice things up. This is a lesbian relationship btw.