Wever portrays Dr. Denise Cloyd, a character from the comics. According to EW, Denise’s role as a medic becomes critical in Alexandria after the death of Pete. She played an important role in some of the books and may prove to be just as integral to the series.
Embry plays Carter, who isn't exactly taken straight from the comics, although showrunner Scott Gimple claims he's a combination of a few different people from the books. (You can see more of Embry in action in the Season 6 trailer.)
Finally, Hawkins will be playing Heath, who is a significant character in the comics. Heath is a supply-runner for Alexandria; as EW notes, Heath eventually becomes romantically linked to Denise. Whether or not that will happen in the series remains to be seen.