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.

Per: https://www.tuhs.org/pipermail/tuhs/2019-September/018452.html

UNIX Release 3 which would go on to be the basis of System III was envisioned as the next PWB release, PWB/UNIX 3.0. However, marketing decided that the name should be made less intimidating to non-programmer users and opted to simply rename this as System III externally. It is obscure, as of this writing, why the PWB name was dropped internally in favor of the generic “UNIX Release 3.0”.