Be very careful with this

Threesomes sound great but can have lasting negative affects on a relationship

I myself personally have never particpated in one, but I know couples that have to mixed reviews

For some couples it's a fun thing and it's done and over with, nobody is really affected and they are able to keep things in perspective

For others it has lasting negative affects afterwords, jealous, anxiety, etc

If you decide to do this with your boyfriend please set clear boundaries and make sure both of you are 100% sure this is what you want, set boundaries for during play and stick to those, make sure everyone has a clear understanding of those boundaries

It sounds like their may be some relationship anxiety issues you may want to work out before doing this.

I wish you the best