Unexpected threesome

i' just came back from a evening with a couple of good friends, tha has ended in an unexpected way.
At about 20H30 he text me to know what i am doing tonight, telling me he is having a dinner with his girlfriend, asking me if i want to crash at their house after that, suggesting that heavy drinking and party will be involved.
As usual, i'm a big party guy so my answer is "Of course, count me in ". I get to their house at about 22h30, we have some drinks, joking and having fun in their couch, the ususal stuff until about midnight, he goes to the kitchen for a refill of our three glasses, at this momment, she was sitting besisdes me, she commes to me french kisses me and put her hand on my crotch just as he came back from the kitchen saying "at last"
A real good night has followed, thanks to them.