While most venture investors won't take cash compensation, there are definitely scenarios where it would make sense to pay them $50K. And, of course, they have the right to set their own compensation (assuming of course they don't violate their fiduciary responsibility to the company).
But since they, as majority equity holders, are accreting value from their decisions as directors, you could make an argument that they are double-dipping if they get additional compensation.
If that fails to dissuade them, then if the other two directors argue that $50K is fair annual compensation despite their equity positions, then I'd extend their principle to the third director (let's assume she's the President). Therefore the President's cash compensation and equity package should be bumped to fair market. That is, whatever the comp+bonus+equity package would be for a new outside President hire, should be given to this third director. The fact that she holds > 20% equity isn't material according to the other two directors' compensation principle.
So let's say that's another 8% over four years and another $100K annually. Now the two directors are looking at pulling $200K plus ~ 2% annual dilution from the company.
Maybe that pushes the impact of their demands beyond what is reasonable for the company and they back down.
Compensation is always a tricky subject. Good luck!