X.3 is an ITU-T standard indicating what functions are to be performed by a Packet Assembler/Disassembler (PAD) when connecting character-mode data terminal equipment (DTE), such as a computer terminal, to a packet switched network such as an X.25 network, and specifying the parameters that control this operation.

In arithmetic and algebra, the cube of a number n is its third power — the result of the number multiplied by itself twice:

n³ = n × n × n.

X3 may refer to:

• , also known as X3 and X-Men 3, the third film in the comic book film series
• X3, a collection of three science fiction novellas by Gary A. Braunbeck
• , a videogame prequel to X-Men: the Last Stand
• X³: Reunion, a space simulation computer game
• X³: Terran Conflict, a sequel to X³: Reunion
• Megaman X3, the third game in Capcom's Megaman X series, a platforming game
• X3 (album), a 2004 solo album by Japanese pop singer Aya Matsuura
• X3 (group), a Chicano rap group