This is an interesting situation. If your relationship was just a standard monogamous relationship my advice would be much different. But the fact is you both set up tinder accounts to look for a 3rd -- I think that changes the dynamic a bit.

Personally I believe when you open the door to this in a relationship it takes a lot of self discipline for both of you to not break what is now a very fine line in terms of trust. Looking at this in an empathetic way, I know I'm imperfect as fuck, so yeah, I can easily see myself making this type of error in judgement. It's not a huge error from what I can tell -- she basically just flirted with someone she didn't know was you.

I don't know. I guess my thought is to do exactly what you have done - which is to confront her. I think going forward you need to make explicitly clear the rules to how you go about meeting people.

Also - one last thing, how do you approach women on Tinder when promoting yourself as single in order to set up a threesome? If your gf were to see any of these conversations would she feel the same as you?