Here is a simple 3x3 door, but extremely quick, created by Mumbo Jumbo. I share with you this efficient door : there is no gravel, no sand and other gravity block ! It's a TRUE 3x3 door with a dobble extender pistons.
It works on 1.6.4.
Be carefull : this door have a speed action, so, the mecanism could be failed by a LAG (if your ping is too hight).
Good Luck & Have RedStone !