Clearly, you are not being valued in this company. At best they are looking at you as cheap labour for next 3-6 months. At worst, you are a founder and a founder leaving is going to raise questions during funding. (Thats why he wants you to leave after 3-6 months after you have raised and not before).

Did you have a launched product/customers when the CTO joined? From your post, I gather that is a no. If so, then your CTO feels you don't deserve your stock. Him and your CEO are trying to cut you off.

Your CEO has not supported you either. He should have been the one talking to you. Not someone you hired unless the CTO has more stake than you which seems unlikely.

If he really thinks you are no longer the right fit for the company, he should have offered to vest you for your money and the time you have put in. And if your work is "not good enough" they should be asking you to leave immediately and not when it is convenient for them. You can't be both needed desperately and not good enough at the same time. To that end, I feel your CTO's ask immature, short sighted and greedy to say the least.

Talk to a lawyer, figure out a deal you could be okay with, try and get that and leave. Them yet to raise another round is a good bargaining chip for you.