Arthur Blundell Sandys Trumbull Hill, 3rd Marquess of Downshire KP (8 October 1778, Hanover Square – 12 September 1845) was an Irish peer, styled Viscount Fairford from 1789 until 1793 and Earl of Hillsborough from 1793 to 1801.

He was born in Hanover Square, the eldest son of Arthur Hill, 2nd Marquess of Downshire and his wife Mary Sandys. He became Marquess of Downshire on the early death of his father in 1801. He was educated at Eton and Christ Church, Oxford, gaining his MA in 1809 and a DCL in 1810.[1]
In 1811 he married Lady Mary Windsor, daughter of Other Windsor, 5th Earl of Plymouth. They had five children:

Arthur Wills Blundell Sandys Trumbull Windsor Hill, 4th Marquess of Downshire (1812–1868)
Lady Charlotte Augusta Hill (30 June 1815 – 24 November 1861), married Sir George Chetwynd, 3rd Baronet and had issue
Captain Lord William Frederick Arthur Montagu Hill (10 July 1816 – 18 Mar 1844), Scots Greys, killed in a hunting accident
Lady Mary Penelope Hill (3 September 1817 – 15 July 1884), married Alexander Hood, 1st Viscount Bridport and had issue
Arthur Edwin Hill-Trevor, 1st Baron Trevor (1819–1894)

During his early political career, Downshire was identified with the Whigs and supported the reform of Parliament. After the Grey Ministry came to power, he received a succession of appointments, becoming Colonel of the South Down Militia on 25 March 1831, carrying the second sword at the coronation of William IV on 8 September, appointed a deputy lieutenant of Berkshire on 20 September, Lord Lieutenant of Down on 17 October (a new office replacing the Governor of Down), and finally a Knight of the Order of St Patrick on 24 November 1831. He received an LL.D from Cambridge on 6 July 1835.[2]