what little i know of the storyline...
is that the arnold in this movie is yet another robot (t1 and t2 each featured different arnie-looking, unexplainably-thick-accent-bearing droids).
the previous two were t-800's. this one is an upgraded t-850 (no clue what the difference is. age?)
the morphing t-1000, as explained in t2, was just a prototype skynet used while developing the ultimate terminator **
the t-x is that ultimate terminator -- a perfected design, infinitely more deadly than arnie or the t-1000.
and she's cute!
** as an aside, the t-1,000,000 was the next evolution -- appearing in a brief, but amazingly cool 3d adventure at universal studios
