A3 Meaning of A3 in English A3 /ˌeɪ ˈθriː◂/ BrE AmE noun [uncountable] a standard size of paper, used in Europe and Japan. A3 paper measures 297 x 420 ↑ millimetre s ⇨ A4 , A5 Longman Dictionary of Contemporary English. Longman - Словарь современного английского языка. 2012