The Torchwood stories began with the Torchwood Monthly Range. Initially, the stories were arranged into numbered series, but the numbering was dropped after series 2 (to avoid confusion with the audio fifth season, which continues after the events of the TV show).
On 3 May 2015, a run of six stories was announced, to be released monthly from September 2015 till February 2016. The main arc of the first series is fighting the Committee although some of the stories, like More Than This, are not related to it.
Shortly after the first story, The Conspiracy was released in September 2015, Big Finish announced what was termed "Series 2", further six stories to be released monthly starting in March 2016, immediately after the first six. The Committee arc continued into the second series.
Soon after the second series was completed in August 2016, Big Finish announced on 7 September that the range will continue with six more stories. However, this time they were not to follow Series 2 immediately due to several special releases in between, including the 10th anniversary special The Torchwood Archive, which largely closed the Committee arc.
Around this time, Big Finish stopped marketing the range using six-story series. In particular, the term "Series 3" was not actually used. Although six-story subscriptions and bundles remain available, they are described as "stories 13-18" rather than "Series 3".