I saw my girlfriend earlier tonight sneaking around with another woman outside her apartment. I saw them kissing and then I saw her leave about 5 minutes before I went into her place. This wasn't just a friendly peck on the cheek; this was full on making out.

I am not mad about her doing this, but I do want in on it., ie a threesome. How do I approach this in a delicate manner so that I don't screw it up?