I'm too much a beginner to comment on the feel between GH and GH3, so I'll just talk about the electrical/mechanical aspects of the GH3.
There is a picture in http://www.yamaha.co.jp/product/epiano-keyboard/clp-170pe/sound-touch.html
explaining the GH3 feature. It's in Japanese but you get the idea.
The so-called "velocity sensors" are actually rubber contact switches, not unlike the keys on a calculator or a TV remote. On most digital piano/keyboard there are two such switches, the velocity is sensed by measuring the time it takes between triggering SW(switch) 1 and SW2. The problem with this scheme is the key has to return to SW1 before the keyboard controller can sense the velocity (elapsed time between SW1 and SW2) again for the next down stroke. However, when the key returns to SW1 it also means the key is back to the home position and the tone should be stopped. This is different from an acoustic piano (somebody please verify this, since I've never played one). On an acoustic you can return a key only halfway before you hit it again, and the hammer will strike again, without the damper stopping the string between the two strikes.
On the GH3 there is an additional switch (SW3), so the keyboard can have two velocity measurements, between SW1-2 and between SW2-3. The keyboard can use the velocity data from SW2-3 to generate a new tone, without going back to SW1 (and stop the tone before generating a new one).
I think the GH3 idea can be developed further. With two velocity values one can derive the acceleration of the key, and refine the generated tone further. But there's no evidence to support whether any current GH3 equipped digitals have used such refinement, or whether such refinement would be worthwhile at all.
The GH3 keyboard has 4 weight zones (A1-F3, G3-E3, F3-D5, E5-C7, black keys similar). Don't know how many zones are in the GH (anyone who knows please share your knowledge). The only indicated new parts on the GH3 are the circuit boards, the rubber contact pads, and the stopper felt (the one hit by the hammers). The keys and hammers seem to be identical on GH and GH3. I do know as a fact that GH and GH3 keys are interchangeable.