If I had haemorrhoids, i'd rather eat razorblades that have anal sex. Just sayin'.
As for the threesome idea - if you are having fun with your girl (and from what you've written, you sound like you are!), just keep it for the two of you. You know she has jealousy/insecurity issues - don't play with fire. It sounds like you're on a really good thing with this relationship - this is more than a lot of people have.