Threesome with boyfriend and another girl?

My boyfriend had a threesome before we started dating. I have always wanted to have a threesome but have never been comfortable enough with someone. 
We have been dating for almost five months, and he says that if I find a girl that I like, and he dosent know well, we'd do it.
I am worried I'd get jealous of her, or that I'd start having feelings for the her
I was wondering if any of you have had a threeway with your significant other and someone else, and how your experience was. 

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