Hi Muffledquack, welcome to Scarleteen.
I think a threesome may be a pretty challenging way to experience sex for the first time. I can't say it will never go well, of course, but a lot of people who already have a good bit of sexual experience find threesomes tricky to navigate, and it is likely to be trickier if you're coming at it without much experience navigating these sexual conversations with other people.
When you are being sexual with one other person, there's plenty of communication that needs to be done around things like safer sex, birth control if that's applicable, and being on the same page about your emotional connection and how each person sees their relationship with the other. When you add in a third person, then you have three
pair dynamics to consider, plus the overall dynamic of the three of you; that's a lot of communication that need to happen to make sure everyone's on the same page when it comes to expectations, sexual boundaries, how you'll handle protection, etc.
What I'd suggest is that you take some time to talk to the people involved to get a sense of what everyone's wants or expectations might be. When you say you found out about this, how did that come up? Did this girl talk to you directly? Have you talked to your friend about this? How do you feel about the idea of being with both of them in a sexual context? All of those answers might help point you to your next steps.