I've read through your posts and notice that you said one of the children is yours. You need to fight to get full custody of your child. However, first you need to have a DNA test on that child to be sure the child is yours. Given how she cheated on you, you need to make sure the child is yours. Get a good lawyer and best of luck to you.
My brother got involved with a woman who is psycho. They never married and had 2 children. He had a job that took him on the road from time to time. He was hearing through the grapevine that his GF was cheating on him while he was away. When he decided to break up with her, he got a lawyer and had the kids DNA tested. The tests proved that they were his. In the end, he got full custody
If you can't get the 2 older kids away from her (I would say, sadly, even if you wanted them, you wouldn't likely get them) and the youngest turns out to be yours, fight hard to get full custody.
If you have trouble affording a lawyer, go to Legal Aid or find a lawyer who will arrange a payment plan for you so you don't have to pay everything at once.