I had a FWB years ago and like other PP we spent a lot of time together not just in bed. It was over 12 months before he suggested bringing in his friend, another man. I said it's not something I'm interested in. I knew this friend from our friendship group but he'd never approached me before. We did all have a chat about it when we were out for a few drinks with this friend after I was asked. They asked if it could be discussed even though I'd said I didn't want to initially. I heard them out but still said no, it was respected by both men and never brought up again.
You barely know this man, you had sex once.
This man tried to lovebomb you to manipulate and confuse you into doing anything he wants.
You don't know his friend and he's not planned any meeting between the 3 of you to discuss the event.
He says this person is a woman, you don't know that you've not met her, it might not be one person even.
What if you don't like her, even if you were wildy into women you might not want to have sex with her or even in front of her. What if you are expected to do something you don't want to. What if they want to film it?
You don't know what to expect of the evening, you could be walking into anything good, bad, embarrassing, humiliating, or downright dangerous.
I'll repeat, you barely know this man why would he expect you to have a threesome so soon? Was that ever duscussed before?
There are good swingers but there are some bloody awful ones too, don't put yourself at risk.
If you want to continue with this man and like the idea of a threesome make sure you meet her. Make sure you know what is expected of the event. Make sure you discuss what you would and wouldn't agree to. Make sure you know what contraceptives you will be using and that condoms are changed if he has penetrative sex (anywhere) with both of you.
Keep youself safe.