Here's a breakdown:

* Eons: The largest division of geologic time. There are four eons: Hadean, Archean, Proterozoic, and Phanerozoic.

* Eras: The Phanerozoic eon is divided into three eras: Paleozoic, Mesozoic, and Cenozoic. Each era is characterized by distinct geological, climatic, and biological events.

* Periods: Each era is further divided into periods, for example, the Mesozoic era includes the Triassic, Jurassic, and Cretaceous periods.

So, Cenozoic, Paleozoic, and Mesozoic are the three major eras that make up the Phanerozoic eon, which represents the time since the earliest evidence of complex life.