What is a The three wise monkeys (三猿)?

三猿The three wise monkeys (Nikkō)

See no evil, hear no evil, speak no evil — carved on the sacred horse stable of Nikkō Tōshō-gū. In Japanese lore monkeys protect horses.

The famous trio is one of eight panels of monkeys that follow a life story.
The stable has eight carved transom panels; the see/hear/speak-no-evil trio is the second, and the panels show the circle of life.
Usual reading of the eight panels: mother and child; the three monkeys (childhood); a young monkey alone; looking up at a blue cloud (ambition); a setback at a cliff edge with a friend's comfort; lost in thought (love); a couple facing rough waves; a pregnant monkey, returning to the start.
In Japanese lore monkeys protect horses (and were linked to keeping horses healthy), which is why they decorate the sacred stable.
The shrine keeps a sacred horse (shinme) in the stable; it is not always there.