Wow, a single mother of 3, you must have your hands full! The biggest thing you could probably do for yourself to help your weight is to take time to care about yourself. Not having the money to eat healthfully is very scary and frustrating! I know for me, there were two things that really helped. The first was to realize that I could make better choices for the same money. A huge bag of mixed frozen veggies was cheap! Combining that with a huge bag of wild or brown rice from the "big box" store, could keep me full and happy with only a small amount of meat for a long time. I could change the flavor of the veggies and rice by adding different flavoring like dried herbs (only $1 a bottle at my local CVS drug store) or from my garden in season. Oatmeal can be purchased in large quantities and is a great whole grain for breakfast, cheap too when you don't buy if flavored in those little packages, flavor it yourself with a sliced apple or anything else at hand! Herbal tea makes a great beverage, no calories and cost effective when made in large quantities you keep in the fridge. Water is free! Popcorn is a great snack and can be bought in huge amounts at the big box stores and keeps forever. Get an "air popper" at a garage sale (I got mine for $3) and you are in business(kids love it) There are lots of choices out there. Don't get bound to the idea that the only "healthy" foods are holland peppers and skinless, boneless chicken breasts! (although you can get amazing buys on those at Sam's or Costco too) And of course lets not forget lovely canned beans, a great source of protein and cheap! Can go in salads and main dishes too!
The most important lesson for me to understand however was simply that I gained weight when I was eating too much. It was the sheer quanity of food I ate, not the quality that was mostly to blame. I had to read the back of boxes and research what a "serving" was supposed to be and follow those guidelines no matter what. It got me used to eating normal amounts of food. A serving is not what they give you in a resteraunt! Even if you have to eat some of the "wrong" food for economic reasons, you do not have to eat excessive amounts of them if you don't want to. That sense can really give you a sense of power even if you are trapped economically in having to buy them! Good luck and take care of you!