Difficult to know what to advice without knowing her current eating patterns, but the problem that most mothers fall into is that they tend to eat up all the food the children leave.
Working on the basis that this is her problem The best way to deal with it, firstly, is to squirt washing liquid on each plate as she picks it off the table, that will make the leftovers inedible.
She may also overcater - and then eat what is left. The answer to this, is portion control.Work out just how much the children eat and cook that much. Do not cook an extra portion just in case they want it. If they are still hungry after a meal, a slice of bread and butter is the best solution. If they are really hungry the children will eat it, if it is just a mechanical reaction they will decide not to have it. Freeze single portions of leftovers, or find ways of incorporating them into other meals.
If the children are between 3 - 8, there will be times when all three are at school/nursery. Once she has walked the children to school (if she doesn't, she should do so, if at all possible), she should keep walking and gradually increase the distance she walks and the speed she walks it at.
If the children are all at school, go down to the local sports centre and book on a course, or use the facilities there.
