Release Date: 1982
Released By: AT&T's Unix Support Group (USG)
Source Code: A portion of the PDP-11 code is browsable in the System III source tree
Documentation: included in the source code

From Wikipedia

UNIX System III (or System 3) is a discontinued version of the Unix operating system released by AT&T's Unix Support Group (USG).

AT&T announced System III in late 1981, and it was first released outside of Bell Labs in 1982. UNIX System III was a mix of various AT&T Unixes: Version 7 Unix, PWB/UNIX 2.0, CB UNIX 3.0, UNIX/RT and UNIX/32V. System III supported the DEC PDP-11 and VAX computers.