It involved a number of formats over the years, switching between a single division and regional (north/south) divisions and during certain years there was relegation to Herts/Middlesex 4 (when in existence). Herts/Middlesex 3 folded at the end of the 2013-14 season with most teams being promoted automatically to Herts/Middlesex 2.
From 2000 to 2003 there was a playoff between the runners-up of Herts/Middlesex 3 North and Herts/Middlesex 3 South for the third and final promotion place to Herts/Middlesex 2 with the team with the superior league record has home advantage in the tie. The promotion playoffs were discontinued for the 2003-04 season when Herts/Middlesex 2 split into two divisions (north and south), meaning that the top two sides in Herts/Middlesex 3 North and Herts/Middlesex 3 South automatically went up instead. At the end of the 2003-04 season the Herts/Middlesex 3 South teams had been the most successful with two wins to the Herts/Middlesex 3 North teams one; and the away team had won promotion two times to the home team's one.
Herts/Middlesex 3 North v Herts/Middlesex 3 South promotion play-off results
^Due to league restructuring London French would not be promoted but would remain in the new look, single division Herts/Middlesex 3 for the next season.
^As with London French, Quintin would remain in the new look, single division Herts/Middlesex 3 for the next season.
^No relegation as Herts/Middlesex 4 abolished for the next season.
^As this was the last season of Herts/Middlesex 3 most teams in the division would be automatically promoted to Herts/Middlesex 2 although Pinner & Grammarians and Old Grammarians would leave the league to join Middlesex Merit Table 5 and 6 respectively.
^Despite finishing 3rd in the league Hammersmith & Fulham qualified for the 2000-01 promotion playoff as champions Wasps Amateurs were denied promotion. This meant that 2nd placed Old Actonians went up instead with Hammersmith & Fulham going into the playoff which was usually reserved for the league runners up.
^One of Feltham's titles was for Herts/Middlesex 3 South.