I'm going to break up with my girlfriend 3 days before her birthday
Why would you break up with your girlfriend before her birthday?
Yes, it's just 3 days before her birthday which is not so nice for me to do it.
They are probably looking forward to their birthday. You shouldn't break up at a time like that and hurt them before their day of the year.
I'm going through a lot of stress and I've thought it through.
If you break up with them before their birthday then they might have their day ruined. Breakups can be hard and occupy someone's thoughts depending on how the break up went. If you truly feel there is no alternative, try doing so in a manner that is respectful and as kind to her as possible.