T3 was really just an attempt to milk more money out of the franchise. Cameron refused to do it because in his view, he had already completed the story. I agree with this, as T3 completely messes with the timeline and changes the theme of the first two, as noted above.

The only reason Arnold did it was because Cameron told him to ask for a "Shitload of money" (picked this up somewhere, don't know if it necessarily is true or not).

It's not a bad movie, it just seemed kind of unncessary.

EDIT: Found the actual quote... it's “If they come up with a decent script that you like and you think you can play, do something cool, and they pay you an awful lot of money, you should just go do it. Don’t feel like you’re betraying me or anything else.”