Hey there, mika. I'm going to take a slightly different stance on this than Char did, and you can consider what both of us had to say as you make your decisions.
It is actually pretty complicated to do a threesome well, especially with friends, where you obviously want to protect your friendship with them. Sex with one person, and only one person, already asks for a lot of communication skills, for you feeling pretty comfortable with your own sexuality, and for managing things that you've struggled with feeling scared about before when they haven't been actual in-person issues yet, like STIs, sexual feelings for your friends, and feeling stigmatized because of queerness. Add on more people and there's a need for even more communication, more comfort and more management of all the things we need to manage with one person.
I also want to say that sex with two people could be hot and tasty, but just like sex with one person, it also could NOT be: how sex is with anyone, no matter how many people are involved, can run the gamut from being the most uncomfortable and boring thing of all time to being some of the greatest times of our lives. Just because it's a threesome doesn't mean it will be yummy. It can be any way any kind of sex can be.
Too, have you met this friend's partner? Do you two like each other and feel attracted to each other? Do you two feel comfortable with each other? What do you know about *their* relationship and what it's like? Is it healthy? Do they seem like a couple ready to be sexual with someone else and able to handle that well?
Lastly, and respectfully, I don't think we can know in advance what we will and won't want to do with sexual partners we've never even kissed before. What we might want to do in the abstract can just be SO different than what we want to do on the4 ground.
If you don't already know these two very well already, and you are possibly interested in pursuing this, why not slow it down and at least start with the three of you just spending an afternoon together platonically to get to know each other, and see how you vibe? If you do that and you are all feeling like a threesome might be something you want to pursue, then it makes a lot more sense to me to start talking logistics -- what you might want and what might be a boundary, safer sex, protocols for if and when anyone wants to stop, etc -- then.
I've written a few advice columns over the years for users asking about threesomes. Here are a couple you might find helpful:
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https://www.scarleteen.com/read/sex-sex ... -boyfriend
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https://www.scarleteen.com/read/sex-sex ... -threesome
I think this piece might also be a good one for you:
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https://www.scarleteen.com/read/relatio ... casual-sex