You can't say Mendy, Bertrand and Alves for 1 Sandro, because you could also get Sandro+Alves as Alves is free. It's whether Mendy and Bertrand is worth 1 Sandro
Yes. That statement makes sense. What he is trying to say is that buying Alves won't cost us a penny. So it is irrelevant to count him in. He is not using up any part of our transfer budget.
That's not true though, because Juve wouldn't sell us half their defence.
Sandro or Alves, we'd only buy one, so you could definitely say Alves+Mendy+Bertrand for the price of Sandro.