"iPad HD" makes far more sense then "iPad 3".
Why?
Because this is a minor feature bump, not a whole new iPad. The processor inside the machine is an A5X, not an A6. It likely contains extensions for Siri, but is otherwise identical to the one in the iPhone 4S.
The GPU is probably the same (maybe they've doubled the VRAM- but I doubt they'd have swapped the unit out for anything newer), the RAM is probably the same, the Flash capacities are likely the same.
Everything is the same except the display, and potentially whatever is required to drive Siri if they decided to include that.
It's not a "whole new iPad". It's just the iPad 2 on steroids, like the iPhone 4S is an iPhone 4 on steroids. It makes much more sense to name it the "iPad HD" (or "iPad 2 HD") in this manner then it is to bump the series number, because then people are going to expect a whole new device (similar to what we saw with the iPhone 5 looking nothing like the iPhone 4 chassis wise).
-SC