I've not worked on a DC17 but think its the same idea as DC4/7/14/33 which are more common.
You have two parts in the picture, the ubend which goes underneath and the valve pipe at the side.
The valve pipe is the one I think you have 3 holes for because it switches from suction underneath to suction from the hose. so in the upright position it connects with one hole that allows air to flow through the hose, then as you tilt the handle backwards it lines up with another so the air flows from the head.