C3, C-3, C.3, C03, C.III or C-III may refer to:
- C3 carbon fixation in plants
- C3-convertase, an enzyme
- Complement component 3, a protein of the innate immune system
- Apolipoprotein C3, a human very low density lipoprotein
- ATC code C03 Diuretics, a subgroup of the Anatomical Therapeutic Chemical Classification System
- Castavinol C3, a natural phenolic compound found in red wines
- Cytochrome-c3 hydrogenase, an enzyme
- Haplogroup C-M217, called C3 in older publications
- In human anatomy, C3 may refer to:
- Clinical Cell Culture, a medical technology company
- C03, Malignant neoplasm of gum ICD-10 code
- C3 Collaborating for Health, a health-promotion NGO
- C3: an EEG electrode site according to the 10-20 system
- C3, Command, control, and communications, a military concept
- C-3 (plastic explosive), a plastic explosive related to C4
- C-3, a United States military designation for the Ford Trimotor
- the designation for several German World War I and World War II armed reconnaissance aircraft
- NAG C.III, an engine powering the Gotha G.IV aircraft
- C-3, a U.S. military transport version of the Martin 4-0-4
- Type C3 submarine (disambiguation), a World War II Imperial Japanese Navy cargo carrier submarine
- Spanish submarine C-3
- HMS C3, a 1906 British C class submarine
- Type C3-class ship, a type of merchant cargo ships of United States Maritime Commission "C" design
- USS C-3 (SS-14), a 1909 United States C class submarine
- USS Baltimore (C-3), an 1888 protected cruiser of the United States Navy
- Garda Crime and Security Branch (CSB), domestic security agency of Ireland's national police, also known as "C3"
- C3 Howitzer, a lengthened variant of the M101 used by the Canadian Armed Forces
- C3, characteristic energy, in astrodynamics, a measure of the rocket energy for an interplanetary mission that requires attaining an orbital velocity above escape velocity
- C3, the chemical symbol for
- C3.ai, an AI company founded by Thomas Siebel
- C3 linearization, a computer science algorithm for resolving the order of method resolution in multiple inheritance conditions
- Delfi-C3, a Dutch CubeSat satellite
- Saturn C-3, a 1960 rocket in the Saturn C series
- Chaos Communication Congress, the biggest annual hacker event in Europe
- Chrysler Comprehensive Compensation System, important in the development of Extreme Programming
- VIA C3, a computer processor
- Creatures 3, a 1999 video game
- Crysis 3, a 2013 video game
- Cluster 3, also known as Samba, an ESA satellite
- Aeronca C-3, an American airplane
- Bavarian C III, a German steam locomotive model
- Bavarian C III (Ostbahn), an 1867 German steam locomotive model
- Chevrolet Corvette (C3), the third production design of the Chevrolet Corvette
- Circumferential Road 3 or C-3, an arterial road of Manila, Philippines
- Cierva C.3, a 1921 Spanish experimental autogyro
- Citroën C3, a car
- Cowin C3, a car
- LB&SCR C3 class, a British LB&SCR locomotive
- London Buses route C3, a Transport for London contracted bus route
- C3 (railcar), a bi-level railcar built by Kawasaki Railcar for the non-electrified branches of the Long Island Rail Road
- Nimrod NRA/C3, a 1983 Group C racing car never achieved
- Yamaha C3, a 2007 liquid cooled 49cc four-stroke motor scooter
- Two different ring expressways in Japan that are numbered C3:
- C3 (classification), a para-cycling classification
- C-3, Nickelodeon's old name
- C3 (light novel), a 2011 Japanese light novel series and anime by Hazuki Minase
- C3, a New Zealand logistics company owned by Linx Cargo Care Group
- C3, the code for permission to use specific land or premises for dwellings in town and country planning in the United Kingdom
- C3, the UEFA Europa League in football
- C3, a standard paper size defined in ISO 216
- Tippmann C-3, a pump-action paintball marker that uses propane gas
- C3 Church Global, a worldwide Pentecostal denomination
- C3, a class in FM radio broadcasting in North America
- Argus C3, a 35mm rangefinder camera
- Nokia C3 (disambiguation), various Nokia cellphones
- Caldwell 3 (NGC 4236), a barred spiral galaxy in Draco
- C3 policing, a methodology being employed by police to combat gangs in Springfield, Massachusetts
- Nielsen ratings that include recorded programs watched three days later
- Championing Community Children, a Philippine charitable organization abbreviated as C3
- Charles Keating III (born 1955), American former swimmer and real estate executive
- Charles III (born 1948), King of the United Kingdom and 14 other Commonwealth realms since 2022.
- C-III Capital Partners, an asset management and commercial real estate services company
- Hexadecimal number C3 (decimal 195)