What do the Japanese say 123?

せーの

The etymology of "せーの" should be the French hisser (pronounced イセー, note: the H at the beginning of the word is not pronounced in French). It means "to lift up."

In the Meiji era, when the government established the army, it referred to the military models of European countries. Among them, the navy’s reference is France. When a French navy sailboat is hoisting its sail, it will pull the rope while shouting "イセー, イセー" (raise, raise). Japan adopted it as a naval term. It is believed that when the word was spread to the general public, it was interpreted as "used when everyone is doing something together", and then gradually changed along the pattern of "イセー, イッセー, いっせーの, せーの".