My girlfriend and I are going to be 3 hours late, and I'm making it happen
Why would you purposefully be late?
I love to spend time with my girl and I thought we could spend this time before the event together, alone.
I understand that you would like to be with your gf, but it is no excuse for showing up late. Being late to an event creates an interruption for others trying to enjoy it.
It should be okay as long I show up on time for the next event.
Your girlfriend might not mind missing tonight's event, or being late to it, but it is still inconsiderate to the other people in attendance. If you and your girlfriend both prefer to be alone, maybe you could just skip this event entirely and show up on time to the next one.