In short: each passenger should be responsible for one-third of the cost of the trip, not for one-third of the cost of the gasoline. Right?

Another factor to consider: Would you take your car if you didn't have fellow passengers? If the answer is yes, you might just split gas 3 ways instead of expenses by 3, and hey, you're saving money.

If the answer is 'no way I'd make a trip like that myself - I'd fly instead' then your proposal sounds reasonable.