Can a defensive player use his hands to grab hold of the offensive player and throw him down and not be called for holding?

Answer 1

It will be obvious that this is foul, and when you are playing in the NBA, even if the referees don't see it, the crowd would've see it and the other team would also see it.

Answer 2

No, it would be holding (if he was caught)

Answer 3

Only if the officials don't see it!