What do three short blasts of a boat horn indicate?

Three short blasts of a horn indicate that a boat is using stern propulsion and is in the act of backing up.

actually engine-room telegraph use- depending on circumstances- All Back Two Thirds ( ship is going into reverse) or if stationary, All Stop. sometimes transliterated as ( Shut her down) or cha cha cha. It is used in this fashion in the song (Little Darling) which mimics and engine- room telegraph.

It means literally, "change of status" as related to a ship's or boat's motion. For example, when beginning to get underway from a pier, the ship will blast three short blasts when the command is issued to the engine room.

Rules of the Road. Rule 34, Maneuvering & Warning Signal

International. "I am operating astern propulsion."

Inland. (Same)

It's a salute upon leaving a port of call. "We're underway!"

"I intend going astern"