A3 n (Printing, Lithography & Bookbinding) a. a standard paper size, 297 × 420 mmb. (as adjective): an A3 book. See also A sizes A3 abbreviation for (Telecommunications) text messaging anytime, anywhere, anyplace Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014