Codes And Conventions Production

 Documentaries are factual pieces of media, often focusing on an event, history, a location, or particular "interest", and much more. Because of their nature, they can often be about very niche topics. 

Documentaries by their nature must be informative- they are meant to be factual and interesting to the audience.  They are generally single stranded, linear structured pieces of media that depending on the topic choice and broadcaster range between 30-60mins in length.
Documentaries often use narration and/or a presenter in order to guide and educate the audience. A presenter may be used on location where this fits with the narrative, while narration may be used over the top of the footage in order to narrate the relevant story/information without breaking up the footage. 
The introduction of a documentary is there to introduce the ideas, concepts or information that will be developed on- Depending on the type of documentary, it may also introduce arguments for the audience to consider. 
The central part of a documentary is where most of the information will be developed and presented to the audience. The purpose is to elaborate and further explore what was introduced in the introduction.
The ending of a documentary is where everything presented thus far is summarised- any conclusions are made, or any ending questions are left open for the viewers opinion. 

BBC Three is a public service broadcasting channel, owned and operated by the BBC in the United Kingdom. 
The BBC as a whole, because it is paid by the taxpayer through the TV license, must have content for every demographic within the UK which has to be unbiased and is not allowed to have any political affiliate. The BBC does this through several channels, each of which are aimed at a different age demographic, much like the similar approach to BBC Radio channels. BBC Three is the BBC's answer to appealing to a teenage and early 20's audience, which arguably also appeals to people up to the ages of 30-35. Because of this, the programs are often present more challenging topics, or topics that are very much in the youth culture of today, an example being "RuPaul's Drag Race"- This being a very open and sensitive topic-based show, that is very present and relevant in today's society. 
Because of BBC Three's "edge" when it comes to show topic choice, it can be the home of many niche topic-based shows, as well as challenging topic-based shows.