I'm so sorry for your loss. Single parenting three kids isn't easy, especially when one is a baby, especially if you don't get a break, especially when they're grieving. If they eat too many takeaways then thats ok.
I'm a single parent to two kids. In our house the solution for Mon-Thur nights is:
Two meals which you all like (or at least which all the kids like), which are very quick to do. This sorts nights with clubs, nights when you're late back from work etc. Ours are beans on toast and a picky plate with sandwiches/crisps/veg/fruit etc. Do these every week. Only swap them for other equally quick meals.
Cook a triple portion of one proper decent, healthy, nice meal each weekend and freeze the remaining two portions of it. Things like spag bol, chilli, sausage caserole, roast chicken, curry etc. Eat two of these freezer meals during the week. This takes a bit longer but is pretty easy to do with the kids around.
I finish early on a friday so Friday after school we often go to the supermarket and choose dinner. The kids often pick quick things like ready meal / frozen pizza etc but sometimes we'll do a proper meal then.
Also, if at all possible book them in for school dinners at lunchtime. Makes life loads easier!