You need to record:

• the names of all individuals of the same surname in the same locality as your ancestors (even the common surnames such as Smith you are searching, if in a small town or township)
• the names of neighbors who may have traveled with them for several generations
• the names of relatives who appear on one record or another

You are doing this because, even though your third great-grandfather might not have stated his parents' names on his marriage record, his sister, who lived in the same county, might have stated her parents' names which was just what you were looking for.