I am divorced and have a fund for my kids. It will probably pay for one full year or two. After that I told them it's up to them. If you agreed to fund 1/3 then it's up to them to figure out the other 2/3. If mom won't help. That's on her and the kids will see what a POS she is.
In this country there is plenty of money available to anyone who want a a education. That's part of why it cost so much. Anyone can get a loan.
Good luck