three-colour

adjective
  1. of, relating to, or comprising a colour print or a photomechanical process in which a picture is reproduced by superimposing three prints from half-tone plates in inks corresponding to the three primary colours
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Examples from the Web for three-colour

Historical Examples of three-colour