The form is trying to figure out if you underpaid Virginia estimated tax. Your zero probably prevented that - which may not have been right.
If you have already filed, do the following:
1. save a PDF copy of your tax return
2. go back into TurboTax as if you were going to amend (don't worry, it won't amend unless you print and mail it)
3. Add the correct number for line 3 on the 760C
4. If the tax due or the refund doesn't change, then forget about it - otherwise you need to finish the amendment process and mail it to the state.
Note: if you save the amended numbers that you enter, then that should carry over to next year.
