Advise for 3some

My gf(24) says she wants to do 3sm a lot. We used to talk a lot about it and she used to cum faster for threesomes and her doing another guy she also wants me to be there while she’s getting the work done, even I want to try but I’m kind of scared about it and I feel like if we start doing it there won’t be stopping anymore, I just don’t know what to do where to start and all and the after effects of doing a threesome. Someone who’d already done it please tell me what it’s like; will we be traumatised after doing the deed??