State tax filing

Form 760C, Line 3 is asking for your 2018 Income tax Liability.  It is possible that this question generated because you had an amount due on your Virginia tax return, as it is calculating an underpayment.

Line 3 is asking for the amount on line 19 from your 2018 Virginia Form No 760.  (This amount represents your 2018 Tax Liability to the State of Virginia)