Threesome turned into cuckolding [MFF]

First of all, sorry for my bad English.

I’ll tell you a story of myself (22)
My girlfriend (20)
And my buddy (23)
I: B
Girlfriend: C
Buddy: M

So it all started with me talking to my girlfriend about whether she could imagine having a threesome.
She said she always wanted to try it but never got around to it.
The topic made us very horny and every time we had sex we talked about it.
At some point I made the suggestion to do a threesome with our buddy. (M)

It was a Saturday evening. (M) came to us around 8 p.m. and my girlfriend and I were already slightly drunk. We had a lot of fun and (M) started drinking too.

We listened to music and chilled on the sofa in the living room. At some point we got into a mood where everyone was very drunk and we started playing games.