Your garndson is lucky to have you!  Not having the mom in the picture on a regular basis is tough, but at 3 years of age some of the most important things that you can do for your grandson are predictability and love.  He needs to know what to expect, so routines are nice.  Also, do not be afraid to use discipline, rewards and punishments for certain behaviors and positive reinforcement (be consistant though!).

Bets regards.