Schedule 3 is for Nonrefundable credits.

A 401k is a qualified plan.

The Savers Credit is for people who contribute to retirement plans and meet other qualifications  (See What is the 2019 Saver's Credit? - Community )

1) Schedule 3 is Additional tax payments and credits (see screenshot).

2) 401(k) plans are qualified.

3) Savers Credit can add to your refund if you contributed to a retirement plan and meet the income requirements.