3U

3U or 3-U may refer to:


 * Sichuan Airlines's IATA code
 * A type of Rack unit
 * Zerstörergeschwader 26, from its historic Geschwaderkennung code with the Luftwaffe in World War II
 * A CubeSat measuring 30×10×10 cm
 * The acronym for the third (3rd) catalog of Uhuru x-ray sources