Let's be realistic, nobody (even hungry teenagers) wants a big elaborate meal every single night, and surely nobody would expect you to go to that much effort.
So I'd say one night put some big baking potatoes in the oven and serve them with a range of easy and not too expensive fillings/accompaniments - grated cheese, butter, chopped spring onion or ordinary onion, sweetcorn tuna, baked beans, mayonnaise, coleslaw, sausages maybe - you get the idea.
Another night, pasta with pesto and peas, salad on the side, garlic bread if you can be bothered.
Third super easy option - egg and chips. Or omelettes.
Fourth option - soup (buy it if you prefer), bread, cheese (or toasted sandwiches).
Fifth option - hummus, falafels, nice Greek/Mediterranean salads/dips/things in jars like olives, pickles, baby peppers and artichokes, decent bread/flatbreads/pittas.
I hope your guests are going to pay for a takeaway or take you out for a meal at least once? Three weeks is a long stay!
Other easyish options that you can prepare all or in part ahead of time or buy ready-made (which will often work out cheaper, tbh):
Lasagne
Shepherd's/cottage pie
Fish pie
Fishcakes
Good luck!