Raptor 3 was the designation of a Raptor deployed to scout the planet Kobol in the episode Kobol's Last Gleaming, Part II. It was destroyed with all hands (including its pilot, ECO, an additional eight crewmembers, including at least four colonial marines) by collision with a Cylon Raider almost immediately upon entering orbit.
In the episode "Fragged", Capt. Lee Adama mistakenly refers to the crashed Raptor 1 as Raptor 3.