Release Date: March, 1977
Released By: UNIX Support Group
Source Code: usg_pg_issue_3.tar.bz2 in the Unix Archive, browsable in the Unix Tree
Documentation: The UNIX Programmer's Manual's for PG-1C300 has been preserved in the Unix Archive
A WIP reconstruction of the manpage sources is maintained here

UNIX Program Generic Issue 3, otherwise known as PG-1C300, is a version of the UNIX operating system issued by the UNIX Support Group in March of 1977. The Program Generic line was used as a standard, supported UNIX base on which to base other projects and supply a computing environment to a larger group of users than the Research group could support. PG 3 would go on to serve as the base of the real-time MERT Release 0 system.

One particular change is the adoption of a new runlevel-based init system, an ancestor of the inittab-driven systems found in later UNIX branches. Issue 3 also sees the adoption of SCCS from the PWB project as well as half-duplex terminal support, allowing for the use of various Dataspeed (Teletype) Model 40 terminals.

On April 25th, 2026 it was announced that a copy of Program Generic Issue 3 had been successfully archived, preserved, and resurrected in SimH's PDP-11 emulator. More can be found here.

See Shamim Sharifuddin Pirzada's A Statistical Examination of The Evolution of the UNIX System for more information about the history of USG UNIX.