Question: What is the warfare branch that includes the No. 3 Operational Training Unit (3OTU), and was formed March 1921?

Context articles: No. 3 Operational Training Unit RAAF
No. 1 Operational Training Unit RAAF
No. 76 Operational Training Unit RAF
Royal Australian Air Force
No. 8 Operational Training Unit RAAF
No. 5 Operational Training Unit RAAF
No. 6 Operational Training Unit RAAF
No. 3 (Coastal) Operational Training Unit RAF
No. 4 Operational Training Unit RAAF
No. 7 Operational Training Unit RAAF

No. 3 Operational Training Unit (3OTU) was the Royal Australian Air Force's main seaplane training unit during World War II.
No. 1 Operational Training Unit (No. 1 OTU) was an operational conversion unit of the Royal Australian Air Force (RAAF) during World War II.
 Formed in December 1941 at Nhill, Victoria, it relocated to Bairnsdale in mid-1942, and then to East Sale the following year.
 No. 1 OTU's primary role was to train aircrew for multi-engined aircraft operations.
 At its peak of activity in August 1944, it was operating over 130 aircraft, the most numerous being Bristol Beauforts. Its aircraft and personnel also conducted transport missions in New Guinea and maritime patrols in southern Australian waters.
 Following the end of hostilities, the unit was disbanded in December 1945.
No. 76 Operational Training Unit RAF (76 OTU) was a Royal Air Force operational training unit during the Second World War.
The Royal Australian Air Force (RAAF), formed March 1921, is the aerial warfare branch of the Australian Defence Force.
 It directly continues the traditions of the Australian Flying Corps (AFC), formed on 22 October 1912.
 The RAAF provides support across a spectrum of operations such as air superiority, precision strikes, intelligence, surveillance and reconnaissance, air mobility, and humanitarian support.
No. 8 Operational Training Unit (8OTU) was an operational training unit of the Royal Australian Air Force formed at RAAF Station Narromine, Narromine, New South Wales in 1942.
No. 5 Operational Training Unit was an operational training unit (OTU) of the Royal Australian Air Force.
 It was formed at Wagga Wagga, New South Wales, in October 1942 to train pilots and navigators for service in World War II.
 The unit was initially equipped with Bristol Beauforts and Beaufighters, and later received Douglas Bostons and de Havilland Mosquitos, among other types.
 No. 5 OTU was transferred to Tocumwal in October 1943, and then to Williamtown in mid-1944.
 It was reorganised as a fighter conversion unit flying P-51 Mustangs and CAC Wirraways in February 1946, and disbanded in July 1947.
No. 6 Operational Training Unit (6 OTU) was a Royal Australian Air Force training unit of World War II.
 6 OTU was formed at RAAF Base Nowra on 5 June 1943 to provide DAP Beaufort aircrew with training in torpedo dropping procedures and tactics.
No. 3 (Coastal) Operational Training Unit RAF (3 OTU) was a training unit of Royal Air Force Coastal Command, operating from 27 November 1940 and disbanding 4 January 1944, being absorbed into No. 6 OTU.
No. 4 Operational Training Unit (No. 4 OTU) was an operational conversion unit of the Royal Australian Air Force during World War II.
 It was formed at Williamtown, New South Wales, in October 1942 to train pilots and wireless air gunners to operate Vultee Vengeance dive bombers.
 The school was equipped with Vengeances and CAC Wirraway aircraft.
 Accidents were common in operational conversion units, and No. 4 OTU suffered several fatal crashes during its existence.
 It was disbanded in April 1944, handing Williamtown over to No. 5 Operational Training Unit.
No. 7 Operational Training Unit RAAF (7OTU) was a Royal Australian Air Force heavy bomber training unit of World War II.
 7OTU was formed on 12 February 1944 at RAAF Station Tocumwal in southern New South Wales to train RAAF B-24 Liberator crews.
 7OTU was initially equipped with ex-USAAF B-24Bs but later received new B-24Js.
 At full strength the unit was equipped with 54 B-24s and was responsible for training 28 crews per month.
 7OTU was disbanded following the end of the war.