Bro, you don't bring it up. 3 somes are the type of thing that "just happen"
Planning for one is like planning to have polio... well... not exactly, but you get my point.
If you girlfriends not a total b1tch and you plan on keeping her around you wont even try.
It will lead to the relationship being over as fast as you can say "nipple thruster"
Fantasies are fantasies for a reason, I've had three 3 somes, and trust me bro, they're not all that.
1. If she is willing to have one, it may be a red flag of a sort
2. If you do go along with it, it was cause fidelity and insecurity strain on the relationship
3. She may think has a license to an open relationships
4. Threesomes never work in standard relationships, leave it alone
Do what thou will though, I suppose.