Columns A, B, and C are blank on Form 1040X because the amendment was for the state return, only. But, I recommend you check your entries. Form 760C is only required for underpayment penalties. If at least 100% of the prior year tax paid was paid in the potentially amended state tax year, or if 90% of the potentially amended state tax year was paid in by December 31, then this form does not apply to you and will not change any balance due to the state,
VA Form 760C Line 3 refers to 2019 Income Tax Liability After Spouse Tax Adjustment and Tax Credits.
VA Form 760 Line 3 is a sum of Adjusted Gross Income from the federal return plus additions from VA Schedule ADJ.
VA Form Instructions Resource
