When did the British threepence (3d) coin ceased to be a legal tender?

Following decimalisation, the brass threepence ceased to be legal tender after 31 August 1971.

The British threepence (3d) coin, usually simply known as a threepence or threepenny bit, was a unit of currency equaling one eightieth of a pound sterling, or three old pence sterling. It was used in the United Kingdom, and earlier in Great Britain and England. Similar denominations were later used throughout the British Empire, notably in Australia, New Zealand, and South Africa.

Before Decimal Day in 1971 there were two hundred and forty pence in one pound sterling. Twelve pence made a shilling, and twenty shillings made a pound. Values less than a pound were usually written in terms of shillings and pence, e.g. forty-two pence would be three shillings and six pence (3/6), pronounced "three and six". Values of less than a shilling were simply written in terms of pence, e.g. eight pence would be 8d.