The reappearance of Arnie in all three Terminator films is explained away but the fact that he’s actually playing different versions of the same machine, all modelled on the same man. A machine which, on the evidence of Terminator 3, apparently doesn’t work out quite so much as it used to in the ‘80s.
 
Lesson for McG: The fans love Arnie, even if he gave up even pretending to act years ago and would need a drastic digital makeover to sustain the illusion that the T-101 isn’t a sagging ex-bodybuilder turned politician.