*FOR A VIDEO GUIDE (RECOMMENDED) VISIT THIS LINK*

1. So basically make a 4 high and 3 wide space.

2. Place a command block wherever you want and type in the command:
scoreboard objectives add Door dummy Door

*In mosts of my tutorials I use scoreboards so I will use them again.*

3. Set up two blocks, one to open the door and one to close it.

4. On each of the two blocks place a command block behind them.

5. Once you have a command block behind each of the two
blocks look into the command block that you want to use to
OPEN the door and type: scoreboard players set @p Door 1

6. In the command block you want to CLOSE the door with
type: scoreboard players set @p Door 0

7. After that you need 12 spaced command blocks
(One for each block in the 3x4 space) you need
12 on each side (12 to open 12 to close).

8. On the side that you want to CLOSE the door, type this
command into the 12 commands: setblock (cordinates) (block type)

For example: setblock -2445 65 235 iron_block

9. On the side that you want to OPEN the door, type this
command into the 12 command blocks: setblock (cordinates) air

10. Once you have the total 24 command blocks (12 blocks per side)
you need to make a redstone loop (1 second perferably)
that connects to one set of the 12 commands blocks.

*You need this so the command blocks can constantly test for a player
with the score of either 0 (Door is closed) or 1 (Door is open)

11. On the OPEN side you need to type: testfor @p[score_Door_min=1]

12. On the CLOSE side you need to type: testfor @p[score_Door=0]