I3, I-3 or i3 may refer to:

• Core i3, a microprocessor line from Intel
• i3 (window manager), a tiling window manager
• Triiodide (I₃⁻), an anion composed of 3 iodide atoms
• Intermud-3, an InterMUD network method between MUDs
• Axiom I3, a large cardinal axiom in rank-into-rank set theory

• Interstate 3, a proposed Interstate Highway in the U.S. states of Georgia and Tennessee
• BMW i3, an electric car by BMW
• Inline-triple engine, an internal combustion engine with three cylinders in a straight line
• LB&SCR I3 class, a British LB&SCR locomotive
• Circuito Norte (I–3), a Cuban highway going along the northern coast of the island
  • Vía Blanca (I–3), motorway from Havana to Varadero
  • Autopista Sur (I–3), motorway in Varadero
  • Autopista Havana–Mariel (I–3), motorway from Havana to Mariel

• Japanese submarine I-3, a Type J1 submarine of the Imperial Japanese Navy
• i3 fighter, a conceptual Japanese jet fighter proposed in 2010
• Mikoyan-Gurevich I-3, a heavy interceptor prototype fighter of the 1950s
• Polikarpov I-3, a 1928 Soviet biplane fighter

• 3i, a company headquartered in London, United Kingdom