Line 9x13 pan with layer of graham cracker (may need to break some to make full coverage).
Mix pudding using the three and a half cups of milk. Fold in the Cool Whip. Pour half over the crackers, add another layer of graham crackers. Pour remaining mixture over crackers, add another layer of graham crackers.
Warm frosting in microwave for about 35 seconds (until pourable). Pour evenly over top of graham crackers (spread if needed).