https://allabout-japan.com/en/article/1508/

https://en.thebecos.com/blogs/column/ichiju-sansai-traditional-japanese-meal-setting

What Is Ichiju Sansai?

"Ichiju sansai" (一汁三菜) literally translates to "one soup, three dishes." It is the foundation of many traditional Japanese meals. The three dishes will usually consist of one main dish and two side dishes, offering endless combinations for a varied meal.

Although rice and pickles are always included as well, you will notice that they are not part of the name "ichiju sansai." This is because both rice and pickles are considered an indispensable part of any Japanese meal, which makes it unnecessary to indicate that they will be served as well.

 The Origins of Ichiju Sansai

Ichiju sansai has its roots in the Heian Period (794-1185) and derives from a banquet-style form of serving meals to nobility (Yusoku ryori 有職料理/Daikyo ryori 大饗料理) and also "honzen ryori" (本膳料理) for samurai class during the Kamakura Period (1185-1333). The elaborate type of dining where dishes were skillfully arranged and served to guests on small wooden or lacquered tray tables. These four-legged tables would be presented in sets of three, five, or seven, with the first tray (honzen) holding 1 soup and 3 dishes, the second tray (ninozen) usually holding 2 soups and 5 dishes, the third tray (sannozen) usually holding 3 soups and 7 dishes, and so on.

Over time, this extravagant way of dining evolved and changed into the simpler everyday ichiju sansai meal we know today, containing the dishes of the original honzen: 1 soup and 3 dishes.

https://trf-ny.com/blogs/news/ichiju-sansai-the-secret-healthy-japanese-meal