The 3rd Brigade is a combined arms brigade of the Australian Army, principally made up of the 1st and 3rd Battalions of the Royal Australian Regiment (1 and 3 RAR). Initially raised in 1903 as part of the post-Federation Australian Army, it was removed from the order of battle in 1906 following the restructure of the field force. It was re-formed in 1914 for service during World War I, taking part in the fighting at Gallipoli and on the Western Front in Europe. During World War II the brigade was used in a defensive role before it was disbanded in 1944. It was re-raised in 1967 for service during the Vietnam War and later went on to provide the nucleus of the deployment to East Timor during the Australian-led intervention in 1999. The brigade is currently based at Lavarack Barracks in Townsville, Queensland.

Following the Federation of Australia in 1901, the structure of the new Australian Army was approved in 1903. Included in the new field force were three infantry brigades of both militia and volunteer personnel, each of which consisted of four infantry regiments, three field artillery batteries, a company of engineers, as well as a field hospital and veterinary and supply units in support. The 1st Brigade was raised in New South Wales and the 2nd in Victoria, each consisting primarily of troops recruited from those states respectively, while the 3rd Brigade was based in Queensland from troops based in the remaining states of Queensland, Tasmania, South Australian and Western Australia. Units of the brigade included 9th, 10th, 11th and 12th Australian Infantry Regiments. However, by 1906 the field force had been restructured into just two infantry brigades, along with five light horse and four mixed brigades. As a consequence the 3rd Brigade was removed from the Australian Army's order of battle, and its units became part of the mixed Queensland Infantry Brigade established at that time. In 1912, following the introduction of the compulsory training scheme, the brigade was assigned to the 1st Military District. At this time, the brigade's constituent units were located across south-east Queensland and northern New South Wales including Brisbane, Ipswich, Toowoomba, Warwick, Lismore, Casino and Grafton.

The 3rd Brigade was re-formed in August 1914, as part of the Australian Imperial Force (AIF) which was raised for service overseas during World War I. As part of the 1st Division, the brigade consisted of four infantry battalions—the 9th, 10th, 11th and 12th Battalions— which were drawn from Queensland, South Australia, Western Australia and Tasmania.

The brigade was initially commanded by a British regular officer, Colonel Ewen Sinclair-Maclagan. After being deployed to Egypt where they undertook further training, the brigade was committed to the fighting in the Gallipoli campaign, being the first Australian unit to land at Anzac Cove on 25 April 1915. They subsequently remained on the peninsula until the end of the campaign in December 1915, taking part in a number of battles including the landing at Suvla Bay and the fighting at Lone Pine and Sari Bair. Following the conclusion of the campaign the brigade was withdrawn and returned to Egypt where a period of reorganisation was undertaken. During this time the AIF was expanded by raising new infantry battalions from cadre personnel drawn from experienced units and bringing in newly trained recruits from Australia. Personnel from the 3rd Brigade were used to raise the 13th Brigade, 4th Division.

Following this, the decision was made to transfer the AIF to Europe to take part in the fighting along the Western Front. In March 1916 the 3rd Brigade, along with the rest of the I Anzac Corps embarked from Egypt bound for France. On 23 July 1916, the brigade took part in its first major battle when it was committed to the fighting on the Somme at Pozières, where the Australian 1st Division advanced 1,000 yards (910 m), capturing the village. In December 1916, Sinclair-MacLagan handed over command to Brigadier General Gordon Bennett, who led the brigade for the rest of the war. Later, throughout 1917 and 1918 battles were fought Bullecourt, Ypres, Menin Road, Passchendale, Hazebrouck, Amiens and along the Hindenburg Line before the fighting came to an end in November 1918.

Following the war, the 3rd Brigade was disbanded in 1919, however the brigade was later re-raised as part the Citizens Force (which later became known as the Militia). Initially the brigade was based in Brisbane, Queensland, however, in 1921, following a reorganisation of the Australia's part-time military forces, later the brigade was re-allocated to the 4th Division and, consisting of the 10th, 27th, 43rd and 50th Battalions, it was headquartered in Adelaide, South Australia.

In September 1939, following the outbreak of World War II, the Australian government decided to raise an all volunteer force for service overseas during the war. This decision was based upon the provisions of the Defence Act (1903) which precluded sending members of the Militia outside of Australian territory to fight. Meanwhile, the Militia was tasked with home defence, and the 3rd Brigade was assigned a command reserve role within Southern Command. At this time, units of the Militia were called upon to undertake defensive duties and to provide training for men that were called up for national service following the recommencement of the compulsory training scheme in January 1940. Initially this was undertaken in small blocks of continuous training, however, following Japan's entry into the war following the bombing of Pearl Harbor and the invasion of Malaya in December 1941, the 3rd Brigade was mobilised for wartime service.

Following this the brigade, consisting of the 10th, 27th, 43rd and 48th Battalions, was attached to the Northern Territory Force and sent to Darwin to defend the port against a possible Japanese invasion. Initially, only the 27th and 43rd Battalions were sent to Darwin, while the 3rd Brigade headquarters and 48th Battalion remained in Adelaide, and the 10th Battalion joined the Adelaide Covering Force. In early1943, brigade headquarters deployed north, and the 48th Battalion was transferred to Victoria to join the 6th Brigade.