Well, SSJ3 was implied to be something completely new, but then SSJ2 was implied to be something only Gohan could do in the Cell Saga, so it's impossible to say. However, Goku obviously didn't go SSJ3 while at risk of dying (he WAS dead when he got it), and I can't imagine a dead Goku managing to get angry at anything. So I would think SSJ3 is something unlike SSJ and SSJ2. Of course, I can't begin to explain how Goku got SSJ2 in the first place.
...and this isn't an SSJ3 thread, so I better bring it back to topic. I personally think the current auto-descend on loss of ki is a good enough system, without factoring HP as well. The Saiyans are pretty good at saying Super right up to the end, you know. I bet Gohan's HP was very low at the end of the Cell Saga.