Chances of having a threesome with your gf and best friend are high, if..

Alcohol lowers inhibitions. Meaning you're still you, just no filter. If your gf never wanted a threesome, you'll never get a threesome.

Weed, however, opens you to suggestion. Makes you vulnerable to things you normally wouldn't think to do.

Let's see. There's you and your girl friend. Is the third person male or female ? Is the third person YOUR best friend or your girl friend's best friend ?

If she’d rather have sex with him than you