I'm doing research for a book and am having trouble answering this question definitively. Would common farmers and laborers in Rome during the Civil War period have used the tria nomina as aristocrats would? After the inclusion of the Latins/Social War, they were citizens, so they would have been sorted into voting tribes/gens- leading me to think they would have used the nomen in addition to personal names. I inferred that common nomen would have been used for commoners as well as the aristocracy, but I worry I am missing something. Any insights on this are greatly appreciated. Thanks.
Nope, not tria nomina. That's the aristocrats, as you say. But it's not just having three names. Gnaeus Pompeius Magnus was a commoner and used three names, but everyone knew Pompeius wasn't a name of a gentes. It was a cognomen, not a nomen, that was used as an inherited family name. So his son could be known as Sextus Pompeius Pius.
Real nobodies hardly kept track of their family cognomen. They would be known by a praenomen or a name based like a cognomen (but not one) with maybe a nickname/ekename/ surname like you would see used in the Anglo-Norman period: something based on where they lived or used to live, what they looked like, how they acted, what they did for a living. Felix the Fisher, Octavia the Stork, that sort of thing.
Then there's the rules for slaves and freedmen ...
You can find them all in the Latin chapter of Ingraham's People's Names. There's also an interesting work on the names foreigners (all commoners) chose in Latin on joining the Legions, but I can't reference that right now. I'll add it later.
When you find People's Names at the library, don't just copy the Latin chapter. You'll probably have characters who need Hellenic, Etruscan, Gaulish and even possibly Punic names. All there.