I entered a contribution from a couple, but the contribution doesn't show up on the C-3 report.

The "couple name" must be different than either of the contact 1 or contact 2 names. Sometimes users make the mistake of entering the "couple name" as one of the contact names. For example: the couple name should be Fred and Mary Jones or Fred Rogers and Mary Jones if they have different last names. If you enter the couple name as Fred Jones and contact 1 as Fred Jones, the C3 will drop the name completely as the software doesn't know if you are attributing the contribution to the couple or the individual if the names are the same.