(27M)Caught my girlfriend(23F) messaging a guy behind my back. Advice?

So I walked in the room and I saw my girlfriend quickly swiping to a different app on her phone in a rush like she was scared. I asked her what was up? To her reply “im Snap Snapchatting my sister in-law”. To me the way she behaved was kinda shady so I asked if I could see. At first she outright refused then after multiple requests she caved, opening her phone away from me. My response was to sit beside her so I could see her open it. Now I don’t use Snapchat, never cared for it by the way. When she opened it I noticed that her sister in-law wasn’t even at the top of her list, it was some person I didn’t know. I said that’s not your sister in-law. She gave me some excuse but claimed it was a dude’s girlfriend who uses his snapchat “umm ok sure i’ll play along show me the chat then?” “no” she says, immediately that threw a red flag for me and I asked why and she couldn’t come up with any reason as to why she couldn’t click his name and pull up the chat. The one thing she tried was to tell me that it’s snapchat, it deletes the messages I was like “ok show me the blank screen?” no again so this goes on and on me asking and her just outright refusing. I’ve never been on her phone although she will sneak on my phone all the time. I eventually left the room to calm down and came back a min or two later. She deleted the chat at that time.

Later she tells me that she was talking to a guy and not his girlfriend but it was platonic i.e music and stuff. I don’t believe her on the basis of it it was a platonic conversation why hide it from me you know? Why fight tooth and nail for me not to see the chat? I eventually left to clear my head and I came back and she had left to stay at her brother’s house. At this point I told her that I was basically done in the relationship. The next day she apologized saying that she should have shown me the conversation and she was sorry. It’s too late I say because she deleted the whole thing so I don’t know. She offered to pull the snapchat data on her phone but everyone i asked told me that i can easily be altered.

I’ve been in this relationship for 3 years and she has done something similar in the past ( I caught her messaging and hanging out with another guy before) funny enough she also claimed that it was platonic and they only talked about work and stuff, she hid that for about a month before a friend told me. I’m really at a loss. I don’t know how to handle this. Should I just dump her? help…