On schule D I have capital loss carryovers that are being double counted: wth entries ed on both line 3 and line 6. How do I correct this error?

Yes, line 3 of Schedule D is for current category C sales, not for your capital loss carryover. You can edit that line. Here's how to do it in TurTurboTax Online:

1. Sign in to your TurboTax account. 
2. Select  Wages & Income, then Review, or Pick up where I left off.
3. Select Edit/ Add to the right of Investments and Savings (1099-B, 1099-INT, 1099-DIV, 1099-K, Crypto).
4. Find the section where the sales were entered, expand the section by selecting the down arrow to the right.
5. Select Edit.
6. Look for the Sales Section column and find the line that shows Short-term (Box C). 
7. Select the trash can icon to the right of that transaction to delete it. 
8. Answer Yes, to confirm your delete.