The third series of the British police procedural television programme Line of Duty, was broadcast on BBC Two between 24 March and 28 April 2016. The series follows the fictional Anti-Corruption Unit 12. AC-12 is led by Superintendent Ted Hastings (Adrian Dunbar) who is assisted by his team, DS Steve Arnott (Martin Compston), DC Kate Flemming (Vicky McClure), and DI Matthew "Dot" Cottan (Craig Parkinson). The unit investigates the actions of Sergeant Daniel Waldron (Daniel Mays), who is suspected of wrongfully shooting and killing a suspect. Waldron leads a team of authorised firearms officers backed by PC Rod Kennedy (Will Mellor), PC Harinderpal "Hari" Bains (Arsher Ali), and PC Jackie Brickford (Leanne Best). Lindsay Denton (Keeley Hawes), a former DI, returns from the previous series as an officer formerly investigated by AC-12. Supporting characters include Gill Biggeloe (Polly Walker), Chief Superintendent Terry Reynolds (Shaun Parkes), DS Sam Railston (Aiysha Hart), Joe Nash (Jonas Armstrong), Maneet Bindra (Maya Sondhi), Chief Superintendent Patrick Fairbank (George Costigan), and DC Nigel Morton (Neil Morrissey). The series was created and written by Jed Mercurio, who also serves as an executive producer. Six episodes were directed by Michael Keillor and John Strickland with cinematographer Peter Robertson. Filming took place in Belfast in 2015. The series received mostly positive reviews and was nominated for 13 awards, two of which were won. The fourth series, which had previously been commissioned with the third, was broadcast on BBC One in 2017. Following the series, a fifth was also commissioned due to the third series success.

In March 2014, while the second series was still airing it was reported that discussions were already occurring on a potential third series of Line of Duty.^[4] On 8 April, it was officially commissioned for a third and fourth series by BBC Two.^[5] Series creator and writer Jed Mercurio returned to executive produce alongside Simon Heath and Stephen Wright for World Productions and BBC Northern Ireland, respectively, and producer Peter Norris.^[6]

Cast members Martin Compston, Vicky McClure, and Adrian Dunbar all returned to the series.^[7] Actor Daniel Mays was announced to be joining the series cast as Daniel Waldron, the latest officer to be investigated by the fictional Anti-Corruption Unit 12 (AC-12).^[8] Craig Parkinson also reprised his role as Matthew "Dot" Cottan.^[9] Keeley Hawes, who starred in series two as DI Lindsay Denton, initially stated that she would not return to Line of Duty due to her conflicting filming on the Fungus the Bogeyman television adaptation.^[10] Will Mellor, Arsher Ali, and Leanne Best also joined the cast in supporting roles.^[11] Mellor, Ali, and Best portrayed a group of authorised firearms officers, led by Mays' character.^[12] Mercurio later decided to bring Hawes back to the series as a result of Denton's positive reception and made changes to a script to allow for Waldron's death.^[13] On her return, Hawes stated that she didn't expect to ever return to the series, but was glad to be given the opportunity.^[14] Mercurio also considered not killing Parkinson's character, but believed viewers wanted justice.^[13] Other supporting cast members include the returning Neil Morrisey^[15] and newcomers Polly Walker, Shaun Parkes, Aiysha Hart, Jonas Armstrong, Maya Sondhi, and George Costigan.^[16][17]

Filming began in Belfast in March 2015.^[18][19] Michael Kellior directed the first three episodes while John Strickland directed the final three.^[20] Peter Robertson provided cinematography work.^[21] Filming locations included BT Riverside Tower, McHugh's Bar, Belfast City Hall, and Invest Northern Ireland.^[22] The series consisted of five hour-long episodes^[23] and an extended feature-length finale.^[24] Due to the success of the series a fifth series was commissioned on 6 May 2016, with the fourth and fifth series moving to BBC One.^[25]