Two ways to look at the question, you are asking if MMF threesome is common and secondly whether you should go for it.
In this culture or other cultures also, threesome is not common. Unless its a porno movie, which is made to ignite your sexual desire, or unless the lady is getting money for the service, threesome is not common. Don't believe in Savita Bhabi comics, thats also porno is comics form. I have seen different part of the world, spoken with people, I found they are always comfortable with only two at a bed.
Second part is about you. Its your fantasy to have threesome with your married girlfriend. When a sex occurs, it should be consenting and both should agree to the act. If she is not consenting and being forced to do something, its not good for future sex encounter. If she is not comfortable, dont force. Full stop.
You hava a fantasy to fulfill, have you asked about her fantasy? Maybe she has a fantasy of MMF threesome where you fuck her pussy and the other guy is fucking your ass. If she forces you, would you jump on her idea? Or maybe she has a fantasy of pegging you, like insert some object or her fist on you arse... ouch, doesn't that hurt? If that hurts, dont try anything where consent is missing.
I hope I could explain, if I have failed to, extremely sorry bro.
