Agile process is used when requirements may change frequently.

The main idea of agile is to deliver software in small increments within short time periods.

This helps the team take feedback quickly and adapt to changes.

Statement $1$ is more related to traditional requirement gathering.

Statement $2$ is more related to risk-driven models.

Statement $3$ is the main agile principle.

Statement $4$ is false because agile processes must adapt to changes incrementally.

Therefore, the correct statement is:

$3$ only