U3 or U-3 may refer to:
People U3 (ゆうみ), Japanese illustrator known for her work on Frontwing games including Grisaia: Phantom Trigger
Transportation U3, an underground rapid transit line in many major German/Austrian cities: U3 (Berlin U-Bahn) U3 (Frankfurt U-Bahn) U3 (Hamburg U-Bahn) U3 (Munich U-Bahn) U3 (Nuremberg U-Bahn) U3 (Stuttgart Stadtbahn) U3 (Vienna U-Bahn) Avies (IATA code: U3), an Estonian airline Cessna U-3, the military model of the Cessna 310 aircraft German submarine U-3, one of several German submarines Boeing Customer Code for Garuda Indonesia
Computer technology U3 (software), a design specification for USB storage, created by U3, LLC U3, a speed class for Secure Digital (SD) cards U.3, an improvement to the U.2 storage interface standard
Video games Ultima III: Exodus, a 1983 video game Uncharted 3: Drake's Deception, a 2011 video game
Other U3, an unemployment figure released by the United States Bureau of Labor Statistics Small nucleolar RNA U3, a type of non-coding RNA U3, a definable user mode (like program with preset) on many Nikon DSLRs and MILCs
See also 3U (disambiguation)