Map of the Middle Earth

It can be inserted in the Second or the Third Age. Indeed, the Beleriand is under waters since the end of the First Age, and the victory of gods against Morgoth.
What can we tell about the Second Age? Humans who didn't help the Elves and the gods against Morgoth stayed on the Middle Earth.So they live here and there on the continent.
There are still Elves also, but their kingdoms will suffer high destruction because of Sauron. Same for the Dwarves.
The Numenoreans (humans who helped the Elves and the gods) won the right to leave on a new land, the Numenor island. They finally came back like sailors to help Middle Earth inhabitants. When Numenor is destroyed (becoming Atalante, under the sea) some of them escape the disaster and found kingdoms like Gondor. They continue to fight Sauron.

The Third Age begins with the victory over Sauron - at least it seems like a victory. Kingdoms of Numenoreans, with the time, are destroyed and it stays only Gondor.
The Hobbits suddenly appear and they finally move in The Shire.
The Dwarves are here and there in what stays of their kingdoms.
Many humans are under the command of Sauron. Beside Gondor, only Rohan and few little areas are free from his influence.