Form 760 and 760CG are the same form - the "CG" means "computer generated". Form 760C is a different form that is trying to calculate an underpayment penalty, if any.
Line 3 on 760C is asking for "2017 Income Tax Liability After Spouse Tax Adjustment and Tax Credits" (which is the prior year tax liability plus some).
The Virginia instructions are less than helpful in that they don't indicate which line they want from the 2017 760. In fact, they appear to want you to calculate the "2017 Income Tax Liability After Spouse Tax Adjustment and Tax Credits" yourself. This would be line 19 on the 2017 760, less lines 24, 25, and 27.
So look at line 19 on the 760, and subtract each of the lines 24, 25, and 27, and enter this number in line 3 of the 760C.