First of all, John cameron refused to direct an installation of third terminator due to another three projoects he has to concentrate.
Second, Linda is willing to do it if John directs. since john rejected that offer, and then linda is no interested in working for acting as Sarah. Thirdy, Sarsh's kid was working for terminator 3 but in few weeks, he was arrested for drugs and director decided to fire him in the next two weeks cause he was in a jail for serious charges. So they hired a replacement for Sarah's kid and re scenes ..
