(from the Wikipedia article)
System III was a version of Unix released by AT&T's Unix Support Group (USG). It was first released outside of Bell Labs in 1982. UNIX System III was a mix of various AT&T Unixes: PWB/UNIX 2.0, CB UNIX 3.0, UNIX/TS 3.0.1 and UNIX/32V. System III supported the DEC PDP-11 and VAX computers.
The system was apparently called System III because it was considered the outside release of UNIX/TS 3.0.1 and CB UNIX 3 which were internally supported Bell Labs Unices.
System III introduced new features such as named pipes, the uname system call and command, and the run queue. It also combined various improvements to Seventh Edition by outside organizations. However, it did not include notable improvements made in BSD such as the C shell (csh) and screen editing.
Third-party variants of System III include (early versions of) HP-UX, IRIX, IS/3, PC-UX, PNX, SINIX, Venix and Xenix.
The files here come from a tarball called "sys3.tar.gz" which is floating around the web. This version appears to be for the PDP-11 platform.