So season 3 started in August but the seasonal playoffs are starting on the 11th of September so technically that leaves one month of ladder (only August).

Season playoffs event is always after the season is completed (just like for example the 2017 Championship Finals was played in January of 2018).

So 2018 Fall playoffs (in September, like you say) are based on Season 2 (which ended with July).

See https://playhearthstone.com/en-us/blog/21553830

Also from their FAQ:

2018 Season 1 * : January, February, March 2018 Season 2: April, May, June, July 2018 Season 3: August , September , October, November * December 2017 is an offseason and will not count toward your point total for compet itive Hearthstone in 2018.