My guess is that the whole story of Atlantis was a complete fabrication by Plato. Oral traditions are rarely reliable even over a few centuries. Of course over 3 million years would be totally preposterous. We don’t even know if the australopithecines had language. Maybe something a little more sophisticated than the communications practiced by present-day great apes, but probably way short of human language.
Fossils are not folklore; the three million years figure shows how long ago Antarctica had a temperate climate, not when was it Atlantis.