How do you unlock the cannon in World 3 in New Mario Super Bros. DS? All I need to know is which level and where in that level. Any help will be appreciated! :-)
The World 3 Cannon is unlocked via the secret exit in the 3-Ghost House stage. In the first area, head as far right as you can. Hit the block under the stairs to the left and press the ? switch that comes out.
While the switch is active, head left back up the stairs then down another set of stairs. Enter the left door under the stairs you just went down before the ? timer ends. Grab the red flagpole to unlock the World 3 Cannon.
In the Ghost House 3, go all the way to the right in first area. Hit the brick to reveal an ? switch. Jump on it, and a door will appear, but you need to enter another one. Quickly run back to the previous switch, you'll see two doors next to that switch, and be sure to enter the left one before the hurry music is over, or the door will disappear. If you fail, you still can practice up and try again.