I honestly don't get how you can make that mistake.
I have been using an Air since May of last year, with three finger swipe of course. And I don't find myself wanting it on the iPad.
I think there is a pretty reason it was left out. On a laptop the act of moving your cursor from wherever it was to the back button is cumbersome and a waste of time, since your hand is likely on or near the trackpad it is a simple move to go back or forward.
This is the reasoning, I think, behind many of the gestures. Two finger swipe instead of having tho use the scroll bar or arrows, for example.
On the iPad, since there is not need to move your entire finger across the surface in order to interact with the back or forward button, there is simply no need for 3 finger swipe.
The action of moving you finger would be as fast, or in my opinion, faster than three finger swipe. The reason I think it would be faster is because over time the act of moving your finger to the back button would become motor memory.
Three finger swipe in contrast may require more coordination if the you are holding the iPad, instead of it being propped or on a stationary surface.
EDIT:
Also, why not two finger swipe since it isn't being used in Safari? Does it have to be three?