Schedule 3 is a new form for 2018.  It is where certain non-refundable credits are reported on your 1040 form, such as the credit for child and dependent care expenses, education credits, retirement savings contribution credits, residential energy credit, and the foreign tax credit. 

The 1040 form was completely revised for 2018.  It is now a much shorter form and many line items that used to be reported on it are now recorded on up to six different schedules (Schedules 1 to 6). 

Therefore, if you are eligible and would like to take any of the credits listed above that are reported on Schedule 3, you will need to upgrade to allow Schedule 3 to be included with your tax return.

See a sample Schedule 3 form here: Schedule 3