Thank you for writing. You already know what you need to do. I hope you will listen to your own good instincts and get the help you need to get out of this situation. From what you wrote, you are not getting anything positive out of this relationship: No financial help; No help with your children; No respect.
Since you are supporting the family, you are not tied into this marriage because you are dependent. You needn’t fear being alone because you already are.
I did some research and found that there is a domestic violence program in your city. You didn’t mention physical abuse but you are experiencing “violence” none the less. In your case, the violence is exploitation and control.
Call that center. Get some counseling about how to leave this situation safely. Then get some more counseling to help you understand why you have married three men who didn’t deserve you. Without some therapy, you are likely to do the same thing with #4.
I wish you well.