Unfortunately, I have absolutely heard people say "this datum" and "these data" -- admittedly not "three data", but certainly in the style of a count noun, and

Unfortunately, I have absolutely heard people say "this datum" and "these data" -- admittedly not "three data", but certainly in the style of a count noun, and not in the style of a mass noun. If they used it as a mass noun, they would say "this data", not "these data".