What does it mean if you hear one prolonged blast and then two short blast?

One prolonged blast followed by two short blasts is often a signal used in maritime communication, indicating that a vessel is signaling its intention to maneuver or that it is making a specific announcement, such as departing or arriving. This sequence can also denote a warning or alert to other vessels nearby. In some contexts, such as in a fog signal, it serves to inform others of the vessel's presence and intentions. Always refer to local regulations or protocols for the exact meaning in specific situations.