Girlfriend having sex with someone else shortly after our mutual breakup

Why do you think it’s any of your business?

Sadly this is just kind of normal - people want to go out and feel desirable again, people also want to feel their ex is somehow still available to them or hasn't moved on.
This is a big reason why after breakups people go no contact, and don't talk about things like joining dating apps or 'confess' who you then slept with, it's just generally hurting each other.

I agree, but dont you think something happening after 1 month is somewhat disrespectful to the 3 years relationship?
in my head its like in the relationship my body is hers and her bodys mine so why would she give something that was mine so soon like she doesn't think what we had was special.

To be blunt, no - you broke up and you weren't married or with kids, you can't disrespect that relationship much more than ending it, and the idea of feeling entitled or possessive about your ex's body and that intimacy just ended as well.
Hurtful as it is, I don't think you can really relate it to your relationship, it's not that she doesn't think that time was special, but she does recognize that it's over. As said after relationships end people often do seek out some intimacy and feeling wanted, and it's also pretty normal to prefer if your ex grieved the relationship for ever more and never moved on. It can be heart rending to think of your ex with others, but over time it just becomes the physical reality of things, you really just need to accept and lean into it I think, rather than live in denial and perceived betrayal.
One thing I'd want to know is why she told you about it and how if it come up? This could be her trying to make it very clear (to you both) that's it's over. It could be her feeling guilty and reconsidering she likewise gave away something she didn't want to or tbh she could just be being cruel.

We'd been together for three years since we were 17, and she was a virgin when we started dating. She recently confessed that just three weeks after our breakup, she met a guy at a club, went on a date with him the next day, and then had sex with him. Although we parted ways mutually and discussed the possibility of reuniting in the future, I find myself feeling extremely territorial over her. Despite her anger at me for joining dating apps soon after our breakup, I had made it clear beforehand that I wanted to experience the apps for fun, even jokingly during our relationship. However, I struggle to reconcile my feelings, as I can't equate joining a dating app with her decision to have sex with someone else. Does her actions reflect on past relationship's strength? As I dont want to waste more time with her if our relationship was weak after 3 years.

Thank you, im starting to feel like i'm being possessive/obsessed. So it might be time for no contact.
we were casually texting and joking around and I said the word "baby girl". she then replied "that word is overused now". I knew for a fact that she only sees that word sexually so I instantly felt a certain way and ask her what she meant, she then replied "just in general". So I specifically asked if she had slept with anyone and it took some time before she finally gave up and said she did.

I just find it hilarious when someone who actively used dating apps while in a relationship with someone (jokingly or not it’s incredibly disrespectful to a relationship and a partner to be using dating apps) thinks it’s disrespectful for their ex to move on from the relationship.

Don't ask questions you don't want to know the answer to then. Agree with others that you need to go no contact and move on, your feelings towards this issue (whilst very common after a breakup) aren't appropriate, she is free to sleep with whoever she wants as soon as the relationship ends.

Lots of threads where one person is upset that their ex has moved on ‘too quickly’.
People’s feelings aren’t at 100% right up until the moment they split. They’ve been changing and fading over time. They might remain fond or affectionate to you but they could equally have been ready to date elsewhere long before the actual breakup.
Other than that, agree with everything StriderHort has written, (but they are on PRSOM 🙂 ).

Mate she don’t give a sh** about you move on bro

Thank you, I am starting to think the same way even though she claims she still does

We'd been together for three years since we were 17, and she was a virgin when we started dating. She recently confessed that just three weeks after our breakup, she met a guy at a club, went on a date with him the next day, and then had sex with him. Although we parted ways mutually and discussed the possibility of reuniting in the future, I find myself feeling extremely territorial over her. Despite her anger at me for joining dating apps soon after our breakup, I had made it clear beforehand that I wanted to experience the apps for fun, even jokingly during our relationship. However, I struggle to reconcile my feelings, as I can't equate joining a dating app with her decision to have sex with someone else. Does her actions reflect on past relationship's strength? As I dont want to waste more time with her if our relationship was weak after 3 years.

We'd been together for three years since we were 17, and she was a virgin when we started dating. She recently confessed that just three weeks after our breakup, she met a guy at a club, went on a date with him the next day, and then had sex with him. Although we parted ways mutually and discussed the possibility of reuniting in the future, I find myself feeling extremely territorial over her. Despite her anger at me for joining dating apps soon after our breakup, I had made it clear beforehand that I wanted to experience the apps for fun, even jokingly during our relationship. However, I struggle to reconcile my feelings, as I can't equate joining a dating app with her decision to have sex with someone else. Does her actions reflect on past relationship's strength? As I dont want to waste more time with her if our relationship was weak after 3 years.