Instead, I would have an open, honest conversation, listen from a place of empathy and generosity and seek a compromise.
The truth is, unless your cofounder has the right to terminate your employment, then continue working and try to create as much harmony as possible.
Co-founder disputes are awful and usually result in the death of the company. But being aggressive will only speed up the Demise not prevent it, and if the company implodes that equity is worth nothing anyway.
Which seems like a valid concern.
You don't move the needle back by being kind and compassionate with those sorts of people. You move it back by using whatever leverage you have. We don't know what leverage the OP has, but his only defense is to use it to either trigger a real buyout, to get the other investor on his side, to risk tanking the company, to say "no" in the belief the co-founder won't tank the company in response, or some other option we're unaware of.
In the same way you won't make headway trying to explain to that lowballer at the yard sale why $5 is more than fair, you won't make any headway trying to explain to someone who has come out of the gate with something so starkly unfair as the OP presents why their proposal is unfair. They know.
"Helping the other co-founder get to the decision" is only going to be achieved with leverage given that it's clear they're not looking to find an equitable (hah!) solution. They're trying to get as much control and equity as possible. It may be the OP has no option but to accept it (keeping 3% and moving on). Or it may be the OP has better options. But the point is that the other party has made their intentions known. They haven't come to the negotiating table saying "This is what I want, how can I get you to agree", they've come to it with "This is what I want; accept it", and when provided what the OP wants in return, turned it down, rather than seeking some middle ground or otherwise budging.
Who's at fault matters here, if the cofounder is truly acting in the way suggested and is not willing to compromise then it's critical to get the other investor to see what they're doing. If that leads to the cofounder blowing up and the death of the company happening sooner rather than later so be it.
If it turns out OP is actually not contributing anything then yeah, better to seek some sort of clean exit compromise. The details matter and how much leverage OP has matters. It's impossible to give good advice without knowing that in either direction.
At a minimum the OP should get the one year cliff on exit.
What kind of compromise do you think is available that treats OP fairly that can be gained by performing empathy, compassion, generosity, and active listening?
From the story told, it sounds like the poster might have tried that already.
Cit. "Never Split the difference".
Something important I learned over time. Sure, listen, but don't take a deal that doesn't make you happy.
