When looking at a variety of advance statistics including PER, Plus/Minus, WARP, Win Shares, and other such stats - many of which factor in defensive performance - all 3 players are consistently at or near the top of the list and those 3 are clearly above all other player in the NBA in performance so far this season. You could make valid argument for any one of them to be on top. As I said before, I'd probably put Curry as #1 right now if I had to choose someone just because he's led his team to the best record in the NBA thus far.