The way I see it, SSj1 can be considered an immature state of transformation, and SSj2 is a kind of true SSj form.
But enter SSj3, easily the most mysterious and confusing form. Why the drastic change in looks? Well, the way I look at it, it may not be a real SSj form at all. More of a power mutation of the other SSj forms. We only see two SSj3s in the manga, Goku and Gotenks. One was dead to start with, the other was a fusion. Both bodies were in an unnatural state. I believe that SSj3 is achieved by pushing an unnatural body to its limits, which causes a mutation that may explain the drastic change in appearance. It would also explain why it is so much more difficult for Goku to maintain after his revival... it's not a form that is meant to be achieved in a natural body, so his body simply rejects it. So I believe the likes of Vegeta and Gohan would not be able to achieve this form in their current state.





