YANBU if these couples are just friends. Travel costs are per car, not per group. A and B live in the same town and A has chosen to cover the petrol, that’s entirely up to them. It’s nothing to do with C how A and B arrange their own finances, and it’s far too late for B to change plans anyway. C are overstepping.
The “fairness” argument from C doesn’t hold water, its flawed because
travel costs are per car, not per group, C chose to drive separately. A and B are sharing a car because they live near each other. There is no scenario where C should be paying towards A/B’s petrol, so why would B pay towards C’s? “They treat themselves plenty” is irrelevant. How B spends their disposable income has nothing to do with C. It’s a judgemental comment, not a fairness argument. A has already decided to cover the cost, C is trying to override A’s personal choice. And finally it’s far too late to change the arrangement. B cannot magically rearrange travel the night before.
The real issue is that C is trying to police other people’s finances, C is overstepping, moralising, judging B’s spending, trying to enforce a “rule” that doesn’t exist and creating conflict where none existed. But there is one scenario where this could be something more deep-seated and not as easy to dismiss - sibling rivalry and favouritism.
If the 3 couples are a combination of parents and their adult married kids and partners, I can conceive that there maybe an intimation of favouritism from a financial perspective. So if couple A are mum and Dad, say couple B are their son and wife and and couple C are daughter and husband for instance. Couple C may feel that parents are subsidising couple B and not them. I'm not saying its right, but I can see that arising in certain families.