William Archer Amherst, 3rd Earl Amherst (26 March 1836 – 14 August 1910), styled Viscount Holmesdale from 1857 to 1886, was a British peer, politician and notable Freemason.
He was born in Mayfair, London, the son of William Amherst, Viscount Holmesdale (later 2nd Earl Amherst) and was baptised on 3 May 1836 in St. George's Church, Hanover Square, London. He was educated at Eton and went on to serve with the Coldstream Guards, rising to the rank of captain and fighting in the Battle of Balaclava, the Battle of Inkerman (where he was severely wounded) and the Siege of Sevastopol during the Crimean War.
On his return from the Crimea, Holmesdale was elected Member of Parliament (MP) for West Kent at the 1859 general election. On 27 August 1862, he married Julia Mann (the only daughter of the James Mann, 5th Earl Cornwallis) in Linton, Kent.
In 1868 Holmesdale became MP for the new Mid Kent constituency, which he represented until 1880. He served as chairman of the National Union of Conservative and Constitutional Associations in 1868. On the death of his father in 1886, he became Earl Amherst. Julia died in 1883, and on 25 April 1889 he married Alice Vaughan (née Probyn), widow of Ernest Vaughan, 5th Earl of Lisburne in London.
He died in 1910, aged 74, at his home of Montreal Park, near Sevenoaks, Kent as a result of an operation he received three months prior for a throat infection. He was cremated on 16 August 1910 and his ashes buried two days later in nearby Riverhead. Despite having married twice, the earl died childless and his titles passed to his brother, Hugh.