I wanna have sex with a girl!!

Ess🎀
So I'm 19 and I've never been with a girl but I've always have been curious. Ever since I got a boy friend that curiosity has skyrocketed lol. I really want to have sex with a girl but since I have a boyfriend it would be a threesome or he just watches. But my question is do you girls know an app that is good meeting people for lesbian casual sex??