"We" is a little bit telling here - I assume you're a part of the anglophone world, because that's where a three-course meal is mostly popular. There are a decent number of answers, but it boils down to what you might expect with a lot of anglophone food: our idea of what ought to be started with the French.

I am not an expert at cuisines worldwide so I will not address any of them in depth, but let me start by noting how often three courses is not the norm. Kaiseki in Japan is multicourse. Tapas in Spain is of course its own thing. And across most of the Islamic world, into India and China, three courses (app / entree / dessert) is atypical.

The closer to today we get, the more information we have. But first, a little historical detour, starting with Jefferson, whose affection for French food from his time in France is pretty well-known. One of his state dinners was this: "Rice soup, round of beef, turkey, mutton, ham, loin of veal, cutlets of mutton or veal, fried eggs, fried beef," along with "a pie called macaroni." That was the meal proper. Then dessert: "Ice-cream very good, crust wholly dried, crumbled into thin flakes; a dish somewhat like a pudding — inside white as milk or curd, very porous and light, covered with cream-sauce — very fine." And there were "other jimcracks, a great variety of fruit, plenty of wines, and good." (This account comes from William Parker Cutler, a dinner guest at the white house).

But more formal dinner service really comes into its own in the 1860s - 1910. I've written about Delmonico, widely considered the first restaurant, elsewhere. Before getting to that, though, I'll start with the French, and let's start with the top of the pile: Escoffier. Escoffier has menus for a seventeen (!!) course menu, as follows:

1. Hors d’oeuvre (appetizer)

2. Potage (soup)

3. Oeufs (eggs)

4. Farineaux (rice & pasta)

5. Poisson (fish)

6. Entrée (entry of 1st meat course)

7. Sorbet (which Escoffier describes as flavoured ice water)

8. Reléve (meat course)

9. Rôti (roast)

10. Légumes (vegetables)

11. Salades (salad)

12. Buffet Froid (cold buffet)

13. Entremet de sûcre (sweets)

14. Savoureaux (savoury)

15. Fromage (cheese)

16. Desserts (fresh fruits & nuts)

17. Cafe (coffee)

(Incidentally, you might notice soup at the start of this list and nuts at the end - thus 'from soup to nuts')

Meantime in America, you have Fannie Farmer's cookbook (published 1896). There's an excellent resource called Fannie's Last Supper - both a TV show and a book - by Christopher Kimball, who recreates a twelve-course menu from Fannie Farmer. It's well worth checking out. It isn't that dissimilar from the Escoffier menu.

And the same is true of Delmonico, which is a great resource because we have the menus. Examples of menus served at Delmonico are available here: https://www.gutenberg.org/files/60285/60285-h/60285-h.htm#Page25

A lot more than three courses, right? Our author, Alessandro (Alexander) Filippini, provides:

There is as much system in serving a fine dinner as there is in running a railroad, or in any other business. French dinners are generally served in three main courses, viz., Relevés, Entrées, and Rotis; all the rest are considered side courses. It depends entirely on the taste of the host as to how many main courses he desires served. The author would suggest two relevés, three entrées, and one or two rotis; this could be made an elaborate dinner.

Relevé, entree, and roti are all main courses, mind you.

Then we can move forward a little further to Paris in the 1920s. If you look at Hemingway's *A Moveable Feast* or (delightfully) anything written by AJ Liebling but especially A Good Appetite, there's not much of a conception of a three-course menu there, either.

So how to we get to three courses? One answer is that we didn't. If you look at properly fancy dinners today, there are often more than three courses, and as restaurants get away from classical French ideas, there are a lot more shared plates / etc rather than coursing stuff out. A lot of restaurants have tasting menus with 5+ courses, too.

Another answer is to take the ideas in Escoffier and Fannie Farmer and Filippini and scale them down. Look at the White House dinners for the past couple decades: we're often down to four courses (five if coffee and a cookie count). They have in common is generally this: there's something that resembles a soup or salad, usually. Then there's a main, usually meat. Then there's something sweet. You can see the same trend in Escoffier and Delmonico, above - the meal resembles a pyramid. Start light; grow to heavy; come back down. And that's just what we have today.

Why does it work this way? One part is based around people's palates: that's what people like. There is an idea called sensory specific satiety, which is the food equivalent of the law of diminishing returns: as you eat more of something, you like it less. This is a significant basis for courses: by changing food, you don't get palate fatigue.

Meantime, the historical reason has, I would posit, more to do with the relative cost of labor, which have increased, and that has led to pared-down meals over time, as we don't have the time to make seven or eight courses all that often.

Sources infra.