unlockable hoppers

I put 3 hoppers in column, each one facing the one underneath, and a redstone block next to the one in the center.
Whenever i throw an item/block in the hopper on top, the item/block travel down to the 3rd hopper, without taking care of the second hopper whitch is lock.
This happend each time, wherever i build it, and no matter the way i locked the hopper with a redstone signal 'on'