Robert Griffin III (RG3) cannot appear in Madden NFL 12 because the game released in August 2011, while RG3 was drafted by the Washington Redskins in April 2012.
He first appears in Madden NFL 13 (roster update / base game). If you're playing Madden 12 and want RG3, your only options are:
For authentic RG3 gameplay, play Madden 13 or later.