Oatmeal with butter, brown sugar, and salt.

The above 3 primary ingredients will be cheap, healthy, and delicious when prepared properly. Adding milk and/or cinnamon to taste can improve the deliciocity.

But maybe don't eat it for every meal or you'll be shitting after every meal. Very clean colon though.

the three sisters are very nutritious. corn, beans, squash. add any spices you like, and a good oil (my faves are la tourangelle olive oil and their toasted seasame oil, sold on amazon and not expensive). salt and spices make all the difference.

IMHO, steamed vegetables are right in the middle of the triangle. I've bought a steam cooker and it's a game changer compared to boiling. It's healthier since less nutrients are lost, preserves so much more taste and texture, there's a timer so you can start the steamer and go do something else. Also makes you use less water. I've still got to try steamed fish but I expect it'll taste great.

humus

If you buy local, and go with the seasons, I'd argue it is rather hard to not have all three (cheap, delicious, healthy) at the same time.

You won't have to rely on produce which is optimized for long transports but can tap into fresh, original flavors. Ripe fruits and vegetables from the fields, harvested just this morning. And because they all ripen now, quality maximizes when prize minimizes.

Rice, tuna from a packet, and soy sauce - cheap, delicious, healthy, and easy. You wanna get fancy, you can add some sesame oil, furikake, chop up some green onions, whatever you got kicking around.

Roasted broccoli from the freezer

Herb Chicken on the stovetop

Lemon vinaigrette with garlic (pour it on the chicken and toss the broccoli in it)

Herbed rice, or rice steamed with coriander

Granita (frozen fruit juice and sugar, stirred occasionally for a icy creamy texture, or do coffee and sugar)

All of these work independently, or together they are wonderful.

Most fruit that’s in season would cover all three.

Another one is curry, which is actually real easy to make. I bought a bag of curry powder for a few bucks years ago and it's still just fine. You can get cans of paste too but honestly I can go either way, both are great, and I love that the curry powder is so absurdly cheap per serving.

I just julienne an onion and red pepper, saute for a bit, add a few teaspoons of curry powder, throw in some garlic and ginger, then add a can of chicken broth, and a few drops of fish sauce. I simmer for a while to let it reduce, then add a can of sweetened coconut milk at the end. Also at the end I add a ton of basil. Maybe some other stuff in there too that I'm forgetting, you really can't go wrong with this either.

For protein you can obviously do chicken or something, but if you want to go ultra cheap and healthy, just throw in a cup of lentils to that curry you got going. Give it 20 minute or so and you're in flavor city. I'm always blown away at how insanely tasty it is, like it's just impossibly good. You can add literally whatever spices and flavorings you want and it just gets better.

There's an asian grocery store near me with all these ingredients for super cheap so I can make that curry sauce for literally like $5-10. It's delicious, cheap, super easy, and healthy, if not a bit calorie dense from the coconut milk.

When I was in college, I had the rule of not buying anything that is >$1.50 per pound. This is what I was reduced to (prices may be different now due to inflation and geo area):

1. Apples, oranges, grapes, strawberries when they are on sale
2. Milk, yogurt
3. Pork shoulder, chicken quarters, thighs, drumsticks
4. ground pork, ground beef
5. Carrots, broccoli, potatoes, cabbage (you'll be surprised at how good thinly sliced cabbages taste in a sandwich)

Yes - generally beans are both healthy (33% protein, 33% fiber, 33% carbs), cheap (dried or in cans), and can be pretty tasty, even out of cans, but if not with eggs, as part of a soup (tomatoes + grain + spices + veggies).

Honestly, I think most food can be found pretty cheap, except for proteins. The best bet I think is chicken drumsticks, but even those will add up. Beans are a cheap protein, but it's about just as carby as it is proteiny.

Well-seasoned, air-fried chicken is super healthy and cheap.

Oh boy I've got one! Bonus, it ticks a 4th box - convenient!

Not sure where you're located and there are different brands, so you'll have to investigate for yourself. But the Tasty Bite brand microwaveable Indian pouches to me manage to hit each of these dimensions. They're cheap (-ish, I wanna say $4 per meal?), healthy (probably high in sodium, but if you look at the ingredients list it's all just food - not weird processed and/or synthetic crap), microwaveable and totally delicious. Granted, it's delicious for a microwaved meal...can't exactly compete with a properly prepared Indian dish. But it's easily the best microwaved food I've ever eaten.

And they're vegetarian and sometimes vegan so a small win on the critter ethics too! Can't recommend em enough unless you mean REAL cheap or you're used to eating home cooked Indian dishes on the regular.

Carrots. Same as potatoes. Boil em, mash em, stick em in a stew. Someone already mentioned onions, same idea.

I know your edit says you were thinking about dishes, and I think carrots can be their own dish with very little preparation. I like to bake mine on a sheet for half hour or so at 425f, and they are wonderful on their own. Also so low-calorie you can eat a practically infinite amount of them without spoiling a diet!

Whole grain pasta.

Homemade Popcorn!

Banana.

Chana masala is pretty delicious and I'm pretty sure it's healthy. I think it's mostly chickpeas and vegetables which are both pretty good for you.

The one thing missing from the trinity is "effort". For instance, you could make any Dal, which would fit the trinity, but takes a lot of time. There are books with hundreds of Dal recipes that all taste different and work, too. And this is just one example. Less than a dollar a meal if made in bulk with 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.