When it comes to threesomes, or more, I prefer the method of meeting the other person/persons before hand. Meeting up for a drink, invitation for a film night at your place, something like that. You will get to know the other person and decide if it is going to work or not.
I don't like the blatent approach coming from nowhere, subtle hints and such can get the message across, and then later ask the person if they would be inclined to join you in the fun. But blatently blurting out "I wanna have a threesome with you" is something likely to scare people off! If your partner has already known her for a while then he can talk to her in confidence about it and ask.
A friend of mine and her partner wanted to have a threesome. They phoned me to meet them in a pub, nothing unusual, but before they hung up she said "I've got something important to tell you!" Hrmmm. Could be anything, I hope nothing is wrong.
I met them in the pub and they both looked super serious. They bought me a drink and sat me down at a table and they still looked really serious! I got worried, and then my friend began by saying "We have been thinking about something for a while and we want to ask you something. We have been wanting a threesome and.. well.. We want to have a threesome with you! Congratulations!" Then she stood up to shake my hand! Like it was a friggin talent show! Lol!
It was her idea of a joke (the make it seem like an interview part) but I was quite concerned that this was the best way she could announce it! Lol!