'III' originates from the Roman numeral system used in ancient Rome; the symbol 'I' represented 1, so three repeated 'I' characters denote 3.

'III' evolved from early tally marks and Etruscan notations into the Roman use of the letter 'I' for a single unit; repeating the symbol produced 'II' for 2 and 'III' for 3, which continued into medieval manuscripts and modern usage.

Initially it meant simply 'three units' in counting and record keeping; this basic numeric meaning has remained stable into modern use as the Roman numeral for 3.