Threesome experience

So, my boyfriend wants to get head but with two girls. Like one sucking the dick and the other sucking the balls at the same time. Anybody had a threesome or allowed a third party into their bedroom? If so, how did you handle it afterwards and prepare yourself before it happen? Also, how was it during it? I am really freaking out and very insecure so I do not know how I feel about it. And my boyfriend is not pressuring me or anything either. Thanks for the help btw!