What are the three divisions of the Phanerozoic Eon

The Phanerozoic, the eon of visible life, is divided into three major spans of time largely on the basis of characteristic assemblages of life-forms: the Paleozoic (541 million to 252 million years ago), Mesozoic (252 million to 66 million years ago), and Cenozoic (66 million years ago to the present) eras.

the Paleozoic (Around 541 million to 252 million years ago), Mesozoic (Around 252 million to 66 million years ago), and Cenozoic (Around 66 million years ago to the present) eras. Those are your answers.