Schedule 3 is a new form for 2018. The 1040 form was completely revised and is now much shorter. The lines that used to be reported on the 1040 form are now on six additional schedules (Schedules 1 to 6).
Schedule 3 includes the non-refundable credits of Foreign Tax Credits, Child and Dependent Cared credit, Education Credits, Retirement Savings Contributions Credit and the Residential Energy Credit. Therefore, if you qualify for any of these credits, you need to include Schedule 3 on your return.
Schedule 3