Threesome

So this guy I'm talking to (we aren't dating just having sex) is asking me to have a threesome. I have never had one before so I'm a bit nervous about the idea. I am not 100% against it, I'm just again a little apprehensive because I don't know the other person. I feel a bit pressured by him though. How should I bring this up without making a huge deal about it? Your thoughts please and thank you!!