Group Trip Car Expenses

Hi so me and a group of university friends are planning a trip. We are a group of 15 and decided on driving 3 cars. Only 2 of us were able to put up our cars and we are going to rent the last car. Before deciding on renting the last car, I was thinking the 2 drivers putting up their cars just wouldn’t pay for gas but now my girlfriend is saying maybe we just shouldn’t pay for the car rental but we should pitch in for gas. So important info is that all gas and car rental will be split amongst everyone among all the cars. How should the people putting up the cars split costs with the rest? Thanks guys please lmk how i should think about this.