Once a C3 has been electronically filed, it cannot be deleted from ORCA.  You can “zero” out the C3 and amend it. If there are multiple contributions included in the deposit, you will delete all contributions but one.  You then edit the remaining contribution to “zero” amount and amend the C3. (Transactions>Contributions>Monetary Contributions>Browse.  Select the contribution to be edited and change the amount to “zero.”