Tacos and burritos could absolutely meet this criteria If you're doing them well at home, with the most unhealthy thing being the tortilla itself (You could argue that those are empty calories). Of course that means no sour cream or cheap cheese abuse among other things

My very Italian answer is "pizza bianca" . I guess "healthy" depends, but it's generally OK carbs, it's delicious and with few Euros you can buy quite a lot (enough for a lunch).

Beans! Especially stew with them, you can throw in pretty much anything (veggies, meat, adjust your spice levels...) and once you learn the correct balance it's very tasty and filling.

Popcorn is very low calorie per volume, especially the types without a lot of fat.

Eggs with salt. Boiled, scrambled, any style really

Ground beef and eggs are about the most nutrition packed and inexpensive foods around.

Many have hinted toward it but there is no natural food that meet all three needs. To achieve that you need to mix up several foods.

Rice, Potatoes, and chicken are extremely cheap and probably your best bet.

How is rice healthy? It's just something to keep me full.

I hope feijoada qualifies here. Black beans stewed with various pig parts (snout, ear, feet), served over rice.

Most things are unhealthy because we eat too much of it. For example (fresh) bread is delicious, cheap, and healthy, provided you eat it in moderation. Now if you ate nothing but bread all day you would gain a lot of weight.

Same goes for salt, fat, and sugar. To be fair, part of the reason we tend to eat so much of it is because normally this stuff is rare in nature and we are evolved to seek it, but we've made it so accessible and cheap, that we easily let our natural instincts take over. So that aspect explains your trinity. But it doesn't have to be that way. You can have all three with a bit of self control.

Okra, it's easy to grow. Bugs and squirrels don't seem to bother it. Frozen isn't too expensive

Tinned tuna. Inexpensive, high in protein and fish oils, low in fat and calories.

Probably not great in huge quantities because of iodine, but generally very healthy

Yes but it's kind of a cheat... The food must be free somehow for one reason or another... This, in and of itself, makes it more delicious even if it was delectable to begin with. Also, obviously the healthy part is important first if it's important to you at all.

I think falafel and pita with all the veggies and sauces might be something that fits the bill that you can find easily.