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 )
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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.