I am also a single father with 3 kids, have been now for 4 years. My kids are now 18 boy,15 girl, 10 boy. My wife of 17 years got hooked on meth and started sleeping with her drug dealer. She made our life a living hell for about 2 years until I finally manged to kick her out legally. I have full custody and she is supposed to pay me child support. For a while after she left I felt like you, not wanting to keep them from their mother and making it possible for her to visit them, but every time they visited with her it just reminded them of how messed up she is. If this is what you are experiencing,
do the right thing, take complete control of your children and forget about her, Do not even mention her when you talk to your kids, it will only make them sad.
Above all, do not let her take them into a dangerous enviroment.
Soon your life will seem normal to you and your kids without her. When your kids grow they may want to get to know her again, for now they need your guidance,love, and
sense of security. Do not let this woman cause you or your children anymore grief, close the door for now. Eventually she may clean up and maybe once again be a part of their lives, until then go on with your life and
forget about her.