It all depends on the exact terms of your agreement of course, but your issues are the same as most founders in your situation:
1. Your value your work and believe the company has potential
2. But the startup is still cash-poor since it's not making money
3. So no one can buy your shares at a price you'd like today
Frankly, the only sane option is to wait and sell your shares later, but relinquish control to the remaining two founders. Support them passively. Understand that a fair deal should have you lose some benefits since you are not actively contributing anymore.