A small-press Canadian Traveller magazine published by Mike Jackson from 1986 to 1988. Eleven issues were published. The magazine included adventures, variants, and other helpful campaign supplements (such as statistics and background for new worlds, vehicles, and items).
The first few issues were reprinted due to popular demand with minor changes.
The final (issue eleven) editorial indicated a 12th and final issue would be released, and also a Best of Third Imperium volume printed at higher quality. It appears that neither of these were ever actually released.
Since 2006 Third Imperium can be freely downloaded at the site that you can reach by clicking on the link.