Step 1:

Make a 5x5 square with a 3x3 hole in the middle, make sure that the square is 3 blocks above the ground.

Step 2:

This is happening in the bottom left corner of the 5x5 square. First, create a dropper donut, so that each dropper shoots into the next one, with the top left one shooting its item into the hopper. Then, place a repeater and a comparator in the spots shown.

Step 3:

Place a block, so that the comparator is going into the block. Place a redstone on top of that block, and place 2 more blocks so that it makes the L shape shown.

Step 4:

On the block next to the redstone, place a redstone torch so that it should be facing off the 5x5 square. Then place a redstone torch under it, on another block.

Step 5:

(Now we flipped over to the other side). Put a block on top of the redstone torch, and place the two other redstone torches as shown. Also, place a block on top of the top redstone torch.

Step 6:

Next, place a sticky piston next to the piece of redstone. The piston should extend, and then place another sticky piston on top of the piston as shown. Make sure that you attach a block ONLY to the first piston.

Step 7:

Under the extended block, place a repeater set to 4 ticks of delay. Then, place two pistons as shown.

Step 8:

Place a repeater, set to 4 ticks of delay, so that it is facing the top piston block, and place two blocks behind the repeater with pieces of redstone on top of them.

Step 9:

Next, place a row of blocks, as shown in the picture.

Step 10:

Add pistons in the following way. They should be in the same "row" as the double extender pistons.

Step 11:

Add redstone dust on the top as shown

Step 12:

Now go back, and on the block where you placed your first piston, place another one with a redstone block attached to it as shown.

Step 13:

Now make sure that the extended redstone block is feeding to a piece of redstone. Place a piston and a block as shown in the picture, with a repeater facing out of the piston head.

Step 14:

Now add blocks with redstone on top of them as shown in the picture, you should only need to place one redstone, as the other two are already there.

Step 15:

Circle around to where your redstone torch tower is, and place a piece of redstone on the block that is above the top torch.

Step 16:

Finally, place a redstone torch, so that it powers the repeater when activated.

Step 17: Final Product

Now this should be your final product, place some redstone and a lever and repeater feeding into the bottom right part of the dropper donut. When you flick the lever, it should open or close, depending on what it was before.

Step 18: With Some Creativity...

With some creativity, you can create a secret passage, which would look really cool in a single or multiplayer world. That's it to building a 3x3 piston door!