Machado and Bryant are definitely better hitters. I'll give you Machado since he's also very good defensively. But Arenado's a far better defender than Bryant. To the point of where it might close the gap Bryant has over Arenado offensively. Plus, I think people do sleep on Arenado's offense because Coors. That said, I totally understand putting Machado and/or Bryant being ahead of Arenado as far as 3B goes. You could really make a case of either one. Bryant is an elite bat with okay defense, Machado is a very good bat with very good defense, and Arenado is a good bat with elite defense. You couldn't go wrong putting either one as 2nd best in the majors. It's so close. 2nd best defensively, though... definitely Arenado. Maybe I'm just overrating the impact Arenado's D has in a park like Coors where the pitcher needs all the help he can get from his D.
As far as Rendon goes... ummm, I don't even know what to consider him anymore. He's far better at 3B than he is at 2B, but he's played both positions with regularity so I have a hard time classifying him as a 3B, especially since he's played far more 2B this year. Not to mention Rendon racks up DL time like crazy and is pretty much a walking bandaid. I definitely have Arenado over Rendon, though, even if Rendon is a 3B. It's a big gamble to rely on Rendon being healthy. If he is healthy like last year, he's definitely right up there, if not ahead.