First threesome advice(18F)???

I’ve been talking to this married couple (19, 20)for about 2 months we’ve been on a couple dates and I get along with them both pretty well they’re very nice and nothing weird so far

Recently they’ve asked if I’d want to have a threesome I’m okay with the idea but I’m nervous it would be my first time with a women (yes I’m attracted to women just never got the chance to have a sexual encounter with one, also the wife hasn’t either ) and I’m not that experienced I’ve only had 2 sexual partners also I don’t know why but I feel like it’s going to be weird since they’ve been having sex with each other for years and I’m the new one in the situation

Is this a bad idea? Should I wait it out ? Any advice would help.