My question is why isn't she paying you a full child support? Get a legal separation and make her pay. My thoughts on the rather draconian nature of the CS system aside, men are too nice to women. She owes you a child support payment on three kids and you should raise bloody hell until the state uses every punitive measure it can until you're paid. Equality is a wonderful thing.
By the way--lest I sounded a bit harsh in my reply I'd like to say that both as a mom who was divorced from my kids' dad, and as a mom who supported my dear hubby's kids, I have kept the exact same attitude. No one "owes" me anything!! I am able-bodied and smart and it is completely MY responsibility to care for my own finances. Now, my own ex was a walk away dad who never paid me a dime for "alimony" type stuff--but I did ask for child support because a parent DOES "owe" their children. man was he UPSET that I asked for it too! My dear hubby's ex is also a walk away parent, and she doesn't even pay child support (or didn't anyway). At the time we thought it was wisest to raise the kids well without having to try to battle her and have the kids be used as pawns. So nothing personal but I've bit the bullet TWICE for parents who walked away from their responsibilities! And I've supported families by myself TWICE without the help of someone who was obligated and I didn't just give up and walk away--so I have earned the right to discuss this topic.