Colin / website
Let us look at the facts:
in 2029 on the eve of losing the war, the machines send a T-800 to 1984, a T-1000 to 1992, and a T-X to 2003
subsequently, John Connor and the humans, having intercepted ALL of this information somehow, send Kyle Reese (John's father) to 1984, a T-800 to 1992, and a T-800 to 2003
Now, James Cameron originally envisioned a scene in T2 where we see Kyle being sent to 1984 and then John Connor (JC…hey, wait a minute…) about to activate the protecting Terminator of T2. So there's production art where we see a long hallway of inactive T-800s (which would have been a cool effect in 1992). Regardless of this unused scene, you have to figure that if John Connor had two T-800s at his disposal, he had three (or four or five…). They could have sent TWO Terminators to protect John in 1992. They could have sent Kyle Reese AND a Terminator to protect Sarah Connor in 1984 since she was the most vulnerable then. Hell, if the 2029 John Connor knew that the machines sent advanced prototypes to kill his younger self, he could have sent multiple Arnolds to protect him in 2003 or maybe a 24-7 Terminator bodyguard to hang around years before.
Linda Hamilton is not in T3. In 2003, Sarah Connor is dead. Or is she? In the special edition of T2, we see her in 2029, playing with her grandchildren in a peaceful futuristic Washington DC. Uh-huh. Apparently this is completely ignored in the new movie. Maybe she faked her death and went underground. But that makes no sense as we know she was obsessively set on protecting John.
These are things that need to be considered when making a sequel, especially when everything was *seemingly* wrapped up 11 years ago.
By the way, the Terminator "theory" of time travel works on a different theory than what we see in Back to the Future which is why John Connor does not disappear at the end of T2.