It would have been nice if in 3 we had gotten a nod to the idea that Marty's plan will still work. With Doc picking the coat up off the ground (wearing it that morning in the cool California in November desert?), finding the scraps and getting a look of "You know..." in his head.
Perhaps Marty was thinking.
"It will work out in end. Sure he tore up the letter NOW but he'll piece together later and ultimately wear a bulletproof vest"
BTTF 3 Marty could cause a paradox/more damage if he mentioned the letter again to Doc.
Doc- Oh right! Now i'm going to throw it in the trash after I ripped it up.
*More complications ensue.*