What are the three periods into which interphase is divided?
Interphase is the preceding phase to the mitotic division. It is separated into three periods, G1, S and G2 (the letter G comes from "gap", meaning interval or breach, and the letter S comes from "synthesis", indicating the period in which DNA replicates).
In fact, "gap" is not totally appropriate for the periods immediately before and after the DNA synthesis. The idea of "growth" would be more adequate since in those periods (G1 and G2) the cell is growing to divide later in mitosis.