A class-3 probe was a type of surveillance probe, that was carried aboard Federation starships in the 24th century. These probes were designed to survive a planetary landing and to be virtually undetectable by sensors.
In 2366, when Geordi La Forge was stranded on the surface of Galorndon Core, Wesley Crusher placed a neutrino beacon aboard a class-3 probe on the USS Enterprise-D. The probe was sent to land on the surface of the planet, where Geordi's visor was used to detect the neutrino particles. He was rescued when he found the probe and modulated the neutrino stream. (TNG: "The Enemy")