Thanks for the Star Trek lookback!
IIRC, the story about Season 3 (according to The World of Star Trek, I think) is that Gene Roddenberry told NBC that he would come back and helm the show personally again if they would commit for a full season and give them a decent time slot. And first they would only commit for a half season, but at a good time slot, so Roddenberry said fine, he'd come back--and then they rescheduled it for 10 or 10:30 Friday night, and Roddenberry dropped out. And thus Spock's Brain.