Never been to the Ozarks myself but jet black squirrels are actually a thing - they're a melanistic variant of the grey squirrel. We get them occasionally over here in the UK too, spotted one near Dundee a couple years back and it proper threw me off for a second.
The weird thing with melanistic squirrels is they tend to cluster in certain areas, so seeing three in a week in the same spot actually makes sense. Some populations just carry that gene more than others.
That said three in one week does feel like a lot. Are they all in the same patch of woodland? Would be interesting to know if locals have been seeing them for years or if this is a new thing. Could just be a thriving local population, or a recent spread from somewhere nearby.
Anyone got photos? Would love to see them.