What does Form 1040, Schedule 3 look like and what is it used for?
The Tax Cuts and Jobs Act changed the 1040 and many supplementary schedules were added. Schedule 3 was added in tax year 2018 to report a taxpayer's Nonrefundable Credits.
Starting in tax year 2019, Schedules 3 and 5 are combined on to a single Schedule 3 Additional Credits and Payments. Line numbers have also been rearranged.
2018:
These amounts included the foreign tax credit (Form 1116), child and dependent care expenses (Form 2441), education credits (Form 8863), and retirement savings contributions credit (Form 8880), residential energy credit (Form 5695) and other credits (Forms 3800 or 8801).

2019:
Starting in tax year 2019, amounts on this schedule now non-refundable and refundable credits as well as payments in two parts:
- Part I Nonrefundable Credits: foreign tax credit (Form 1116), credit for child and dependent care expenses (Form 2441), education credits (Form 8863), retirement savings contributions credit (Form 8880), residential energy credit (Form 5695), other credits (Form 3800 or 8801).
- Part II Other Payments and Refundable Credits: 2019 estimated tax payments and amount applied from 2018 return, net premium tax credit (Form 8962), amount paid with extension, excess social security and RRTA tax withheld, credit for federal tax on fuels (Form 4136), other credits (Form 2439 or 8885).
