I agree with k that you might have some issues. Breathe. Relax. Chill.
Treat women with respect and the rest will flow.
Anyway....
If you have a 3-some it can mess up the relationship you have with your existing partner. Even when she says she's "up for it". Maybe she isn't really...
Have a proper chat about it. You'd be surprised.
- Some women are nympos, closet bisexuals and into all sorts of shit.
- Others just want to please their rather demanding man and say what they think he wants to hear.
So, if it is the case you wish to remain on really good terms with your girl, check which of the two above it genuinely is.
(Rebus Tip: #1 is best if you want to stay on good terms with each other;
#2 is still good if you just don't give a monkey's about her.... howver not giving two shits about her isn't very cool)
As for doing the actual arrangements, if it goes ahead:
a) you could ask her if she finds attractive any of the women in your social group of friends and friends' friends
b) you could advertise on Adult Friend Finder or craigslist casual
c) all three to meet up for a pub drink first (in a pub nearest to the flat or house) for a brief get-to-know and alcohol intake session
d) Style's
dual induction massage might be appropriate depending on how you read the situation....
Good luck in your journey.