Schedule 3 has many forms feeding into it. If there is information on those forms, it will not delete. The key is to delete the forms feeding into Schedule 3.
Schedule 3 contains: foreign tax credit, credit for child & dependent care, education credits, retirement savings contribution credit, residential energy credit, and a few more. If you accidentally entered a credit, this is why it is showing up.
Go to Deductions and Credits and see if any of the credits are areas that you explored. Revisit those areas. You might want to check the ones mentioned above.
Another option is to Delete a Form. If there are forms feeding into the Schedule 3, it will not delete. So you must delete those forms with credits that are not necessary. Here is how to Delete a Form:
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