Answer:
According to the diagram, step 3 indicates the S phase of cell cycle, which corresponds to the DNA replication phase.
Step-by-step explanation:
In the S phase of the cell cycle the nucleus contains twice the amount of its genetic charge, due to increased DNA replication, resulting in each chromosome forming two chromatids.
This is a relatively long phase, and is part of the preparation process that occurs before an imminent division. In the diagram it is indicated by the number 3.
The S phase is named for the abbreviation of Synthesis.