I really want to have sex with my bfs friend.

Hi! I've been dating my boyfriend for a little over 2 years. I have already cheated with one of my coworkers and one of my guy friends. Both of them were single though. His friend has had a girlfriend for years but I want him. Everytime my bf talks about him or I hear them playing games I get so turned on thinking about him. I have no issue being shy and getting out there but I've never been with someone else who's also been in a relationship. I've only cheated with single men.

Any advice on how to test the waters? What can I say? I have his Instagram and Snapchat.