[Request] Splitting up dinner tab

I am going out to celebrate both my birthday and a friend's birthday with my girlfriend. Normally when the 3 of us celebrate a birthday it is just one birthday and the other 2 people split the bill 50/50 to treat the birthday person. My question is how would I split up the bill so that it is not just my girlfriend paying the entire bill? If she paid for the whole dinner then my friend and I would not be contributing towards the others share of the bill. I think that my friend and I should contribute some percentage below 33% in order to still treat each other. Thanks.