Okay, I think I was misleading/confusing. By 3 ingredients, I meant 3 actual foods. Corn, soybeans and wheat. The rest were all supplements. Maybe they're all absorbed fully, but I expected a wider mix of grains or seeds if that's their staple for life.
That depends on what you consider a "supplement."
For example, I am confident salt is on that list. Chickens need a certain amount of salt, but the amount is small, so it is way down the list of ingredients. If you are feeding layer feed, it will have a calcium source (oyster shell, or "calcium carbonate," or something like that), but again the amount is small so it will be quite a ways down the list.
Corn and soybeans together can provide the vast majority of what a chicken needs: enough protein, an appropriate set of amino acids within that protein, enough calories for energy, and I believe also an amount of fiber that is appropriate for chickens. The rest of what they need really is small amounts.
I am a bit skeptical as to whether additives can substitute for the real thing.
With chickens, it get tested a lot better than with humans.
People really do put thousands of chickens in cages or a big barn, and feed them nothing else, and pay attention to whether the chickens show symptoms of deficiency.
For humans, scientists usually figure out what is "needed" by looking for symptoms of problems, and asking what the people eat, then making a guess and trying something. Because the people are almost always free to eat other things, this is less accurate than making a supposedly-complete diet and watching what happens when the animal eats nothing else for its entire life.
There are many chickens, in both commercial flocks and some backyard flocks, that eat NOTHING but the commercially-available feed, and do just fine.
@Kiki is strongly in favor of that method, and gets very good results with their chickens.
(It is also true that there are many healthy flocks who eat a more varied diet. I'm not saying commercial feed is the only way to have a healthy flock, just saying it is one way that does work and is quite well tested.)