Math difficulties, pessimism, or what?

Yesterday I said that the Cubs need only win one game of the NLDS to ensure they play Game 4 (to which my friend got tickets). That's not, strictly speaking, correct, and it assumed the only way the series would end in 3 games would be if the Diamondbacks swept the Cubs. My cousin pointed out the flaw in my reasoning. It's possible (however unlikely) that the Cubs could sweep the Diamondbacks, with the same result for Sunday's game.

So, to revise and amend my comments, as long as each team wins at least one game of the next three, I'll go to Game 4 on Sunday.