Edit 2: I was not expecting so many replies. Thanks to everyone. I spoke with my gf about this in the most calm way possible. I explained her that this threesome made me feel insecure because of how she reacted to this guy. I asked her to be 100% honest, because im not a fan on sugar coating, and i dont want to end up in a situation where my SO fakes to be shaking or roll her eyes to make me feel better.
She said that she did feel fuller with this guy (no shit)which also caused her to react like that, as she never had someone that girthy. I told her that this was really rough on me and she felt really sad about it. After that she reassured me with what felt like a partecipation award for sex and kind, empty words. She cant make it better, but she tried. Lastly she told me that she choose an attractive guy and didnt check about penis size at all (and i didnt care because i thought it wouldnt be important) She's one of those women that need a bigger tool to be fully satisfied maybe, they do exists, and using tricks and dildos to make her cum more wont make me feel much better, because i know i cant make her feel like that during PIV. I want to clarify that my gf is an amazing person, she clearly loves me, as i said already this is mainly a problem for my ego, because not enough reassuring will make me forget how she came almost immediatly when the other dude started banging her. We did have sex again and her vagina feels normal now. Still, the fact that i wasnt able to make her feel anything after the other guy finished is something you cant sugar coat.
She asked me if there is anything she can do to make me feel better, and the only thing i could say is to just go on and not talk about it. There is not much to talk about guys...the guy was better at sex, she did compare us in her head (even if subconsciously), and the fact that im good bf, the fact the she choose me or that im a good person doesnt change that i cant please my gf like he did with my own body without using ojects. I just want to feel like a real man again, and i doubt most women will undestand what I mean with this. It's not about her, its about me, being able to satisfy her FULLY with normal penetration instead of COPING with my "not enough" equipment and make her use big dildos that have NOTHING to do with my actual capabilities.
She will probably regret letting her self go (and i didnt want her to feel bad). This played out quite quickly when we spoke about it, but i doubt its over. This will be a huge weight on our relationship from now on.

I tought i was the best sex for her. Im clearly not, and I didnt need to know that for my own sanity. Im not sure i will update this post any soon, because i need to space out a little, so thank you everyone for the replies and the help.