The OH is bi and she fantasises about threesomes whilst we are having sex and in the moment (probably minus me!!!) The thing is that this is just that, a 'fantasy'. It may become reality if it was spontaneous and not something organised. That was my ill thought out rationale behind my suggestion.
Ok, thats a bit more information
Find out what she
likes, blondes/brunettes, short/tall, athletic/full figured etc.
Find out what she
fantasises about, being dominated, being dominant etc.
Book a holiday or at the least a dirty weekend away (this is the key), I have found in the past that girls are more adventurous when away from home, it adds to the excitment, its a trick I have used to get girls to try various things like WG's/WS/Anal, get them out of their comfort zone or anything that grounds them into their noraml daily grind etc.
On the first night you are away, arrange a WG, to come to your hotel/room whatever, arrange it later in the evening when its darker. Make sure its the 'type' she likes.
Take the missus out to dinner early get her merry (its really important, not to let her get pissed) start telling her about a dream you had with her and a girl, doing all the things she previously told you was her
fantasy, tell her "I didnt expect it to turn me on, but it has", this makes her feel its not 'wierd or strange'.
When you are heading back to the room tell her you have a girl booked for her (or say its booked for you and she is welcome to join, if for instance she said she would rather be a voyeur touching a little bit etc). If she stops dead and says 'fuck off' or some such, discreetly call/text the wg and cancel, if she is like 'I dont know....' you are in. Aim to be back at the room at least an hour before the WG arrives, have a hot sexy shower together, fall into the bed, kissing etc and await the knock on the door....
I know its a lot of faffing around, I assure you its worth it, and you won't always have to jump through all thse hoops, if you get it right first time..
Play the long game