“Koshin stones in the precinct of Tokyo reveal three seated monkeys as stated above. According to Meider, they ‘represent the Japanese expression ‘\mi-zaru, kika-zaru, iwa-zaru, or ‘not seeing, not hearing, not speaking,’ in which the Japanese word zaru can also be associated with the word for monkey saru, giving us perhaps a linguistic clue to why it is three gesturing monkeys being portrayed.
“The first English reference to the three monkeys proverb appeared in 1884 in an English travelers’ handbook to Japan … A reference in the British Army & Navy Stores catalog  demonstrates the carvings were being marketed commercially. Florence Boyce David (1873-1938), a Vermont poet, published a four-stanza didactic poem about the three monkeys with an illustration in a children’s magazine , reprinted five years later in a popular anthology.”
– Folkloristics: An Introduction, Robert A. Georges, 1995
From the wiki: “The three wise monkeys, sometimes called the three mystic apes, are a pictorial maxim. Together they embody the proverbial principle to ‘see no evil, hear no evil, speak no evil’. The three monkeys are Mizaru, covering his eyes, who sees no evil; Kikazaru, covering his ears, who hears no evil; and Iwazaru, covering his mouth, who speaks no evil. Sometimes there is a fourth monkey depicted with the three others; the last one, Shizaru, symbolizes the principle of ‘do no evil’. He may be shown crossing his arms or covering his genitals.
“The source that popularized this pictorial maxim is a 17th-century carving over a door of the famous Tōshō-gū shrine in Nikkō, Japan. The carvings at Toshogu Shrine were carved by Hidari Jingoro, and believed to have incorporated Confucius’s Code of Conduct, using the monkey as a way to depict man’s life cycle.
“There are a total of 8 panels, and the iconic three wise monkeys picture comes from panel 2. The philosophy, however, probably originally came to Japan with a Tendai-Buddhist legend, from China in the 8th century (Nara Period). It has been suggested that the figures represent the three dogmas of the so-called middle school of the sect.”