It will for sure bring chaos and drama into your life.(which i'm sure already has some kind of drama in it)..If your the type of guy that can just roll with whatever is thrown your way, then go for it...If your the type of guy that can't roll with things, then still go for it if not for any other reason then principle alone.....
If you don't do it, there is no doubt in my mind that when you two do break it off one day----you will always say to yourself " I should have done it"
People that end there relationships after they have done threesomes and what not ---Don't end them because of the threesomes(although they will use it for excuse to end it)But end them because of things that were already in there relationship to begin with.....This will not break up your relationship--if you go ahead and do this(like I think you should) and you two break it off soon after, its because your relationship was in trouble well before this----Understand that and use it to your benefit... Go with the 3some and don't regret it----peace