There are plenty of places for handwaving and hope in startups already. The ownership status of a departed founder should not be one of them. That should be crystal clear.
This really puts a giant question mark on why anyone would bother with a non-CEO founding role. Startups are hard enough as it is, adding to it the potential of losing all your equity because you had a falling out with the CEO makes it almost not worth the trouble.
Engineers getting into partnerships with domineering business types should definitely watch out for this and evaluate their options. At least when you work for Microsoft and leave after 2 years, they cannot claw back what you earned over that period of time.
That's why you set up terms up front. If, when they decided on a 33/33/33 split, they also decided on a vesting schedule, they wouldn't have this problem. The OP would get some percentage value based on math, from contract terms they all agreed to when they started.
If you're losing all your equity after over a year of work just because you had a falling-out with co-founders (or investors), you didn't do your homework.
So which side are you arguing on?
What exactly is this standard? Haven't heard it before. (serious question)
edit: from the parent:
A typical vesting agreement would have vested your stock over a 48-month period, so after 18 months you would have vested 33% x 18/48 = 12.375%
I would say he should stick to 12%. It's derived from an industry standard. VCs who have a problem with this are, pardon my french, greedy assholes.