Why do they even think Three Billboards is a racist movie? There are three black characters and all of them are portrayed positively
But there's a racist character in it and they unironically think that putting a racist or sexist character in something, even if they are the villain, means you are racist and/or sexist
We'll he's not really the villain (it's not exactly that type of film) : he is trying to redeem himself and he's not deplatformed by the other town people (the fact that in a small town you can't exactly insulate or move at your leisure, which is sort of the premise, is not exactly the sort of headspace ERA is at since 2016) and that's what they take issue with.
To be fair, the police chief is going at quite some lengths to cover for a known racist who commits police brutality on the regular on a limb that the guy may turn decent somehow. Just a slight change of how the story is framed would make it pretty sinister and repugnant.
Edit : though the film made it quite clear that the racism was apparently pretty systematic regardless of how sympathetic the characters may come across.