This draw bridge fits in a 3 block wide bridge, when the button is pressed a 2x3 hole drops 2 blocks down and a 1x3 section on the inside of the hole extends upward to block anything in the hole from getting out. At a second button press the hole is closed and the raised blocks are dropped to create a flush 3 wide bridge.

This design can be submerged, the redstone just needs to be covered.

I used this design in my survival world and it works really well, a little resource intensive but its worth it. mobs never get past and if there are any mobs in the hole when the bridge is raised they are killed.

This design is horizontally expandable and the hole can be moved further from the button.