Ask them to assess when they think the startup will be profitable, considering everything invested to date, then reproportion your equity accordingly. So if each of you have 25%, the consensus is 2 years to profitability, and you spent 2 years there already, then you should agree to reduce your share to 12.5%, splitting the other 12.5% between the remaining 3 founders. You can also offer to play an advisory role for a bit more.