I am biased because I love Steve Carell, but I must admit that as much as I enjoyed the energy and charisma he brought to his role, I didn't find his casting and performance as credible as those of Bale, Adams and Rockwell (although the latter has always been such a no-brainer piece of casting as Dubya that there arguably weren't any real surprises to his performance, in contrast to the superb work done by Bale and Adams).
I get the impression that Carell was cast as Rumsfeld mostly because he is part of Adam McKay's unofficial repertory group (having appeared in the Anchorman films and The Big Short) rather than because of any real likeness and affinity to the part, but like I say, I still enjoyed Carell here. He plays Rumsfeld as an almost impishly amoral figure, like an irresistibly charming devil on Cheney's shoulder (although it's not long before Cheney outdoes Rumsfeld in the amoral stakes), which makes sense in terms of how the film portrays an initially directionless Cheney as being effectively seduced by Rumsfeld's charm into becoming a Republican.
Three Billboards is superb. I've recommended it to many friends and family members and all have reported back positively on seeing the film. I'm pleased Rockwell and Frances McDormand won Academy Awards for that film.