The 3rd (United Kingdom) Division, also known as The Iron Division, is a regular army division of the British Army. It was created in 1809 by Arthur Wellesley, 1st Duke of Wellington, as part of the Anglo-Portuguese Army, for service in the Peninsular War, and was known as the Fighting 3rd under Sir Thomas Picton during the Napoleonic Wars. The division fought at the Battle of Waterloo, as well as during the Crimean War and the Second Boer War. As a result of bitter fighting in 1916, during the First World War, the division became referred to as the 3rd (Iron) Division, or the Iron Division or Ironsides. During the Second World War, the division (now known as the 3rd Infantry Division) fought in the Battle of France including a rearguard action during the Dunkirk Evacuation, and played a prominent role in the D-Day landings of 6 June 1944. The division was to have been part of a proposed Commonwealth Corps, formed for a planned invasion of Japan in 1945–46, and later served in the British Mandate of Palestine. During the Second World War, the insignia became the "pattern of three" — a black triangle trisected by an inverted red triangle.

The division was part of the Allied British and Portuguese forces that took part in the Peninsular War. It fought at the Battle of Bussaco in September 1810, the Battle of Fuentes de Oñoro in May 1811 and the Battle of El Bodón in September 1811, before further combat at the Siege of Ciudad Rodrigo in January 1812, the Siege of Badajoz in March 1812 and the Battle of Salamanca in July 1812. It also fought at the Siege of Burgos in September 1812 and the Battle of Vitoria in June 1813. It then pursued the French army into France and saw action at the Battle of the Pyrenees in July 1813, the Battle of Nivelle in November 1813 and the Battle of the Nive in December 1813. After that it fought at the Battle of Orthez in February 1814 and the Battle of Toulouse in April 1814.

According to Picton, the fighting by the 3rd was so intense at the Battle of Vitoria, that the division lost 1,800 men (over one third of all Allied losses at the battle) having taken a key bridge and village, where they were subjected to fire by 40 to 50 cannons, and a counter-attack on the right flank (which was open because the rest of the army had not kept pace). The 3rd held their ground and pushed on with other divisions to capture the village of Arinez.

The 3rd Division was also present at the Battle of Quatre Bras and the Battle of Waterloo in the Waterloo campaign under the command of Lieutenant-General Sir Charles Alten K.C.B. (Count Carl von Alten).

The 3rd Division took part in the Crimean War and fought in the Battle of Alma and the Siege of Sevastopol. It was under the command of Lieutenant-General Sir Richard England.

During the Second Boer War (1899–1902) the division began under the command of General Gatacre. In 1902 the army was restructured, and a 3rd Infantry division was established permanently at Bordon as part of the 1st Army Corps, comprising the 5th and 6th Infantry Brigades.

During the First World War the 3rd Division was a permanently established Regular Army division that was amongst the first to be sent to France at the outbreak of the war as part of the British Expeditionary Force (BEF). The 3rd Division served on the Western Front in France and Belgium for four years, from 1914 to 1918. During this time, it was nicknamed "The Iron Division". Its first commander during the war, Major-General Hubert Hamilton, was killed by shellfire near Béthune in October 1914. The division served in many major battles of the war, including the Battle of Mons and the subsequent Great Retreat, and later the First Battle of Ypres and the Battle of Passchendaele.

After the end of the First World War, the division was stationed in southern England where it formed part of Southern Command. In 1937, one of its brigades, the 9th Infantry Brigade, was commanded by Brigadier Bernard Montgomery. He assumed command of the 3rd Division shortly before Britain declared war on Germany in September 1939.