Nothing is known about Hunk, Carlos, or Billy. Rebecca is one of the few remaining S.T.A.R.S. members still alive. Her current whereabouts are unknown. My educated guess for Hunk would be seeing as how he's a mercenary he continued his life as normal working for companies that would pay him, or he may still work with whatever little remains of Umbrella that still left procuring virus samples and eliminating threats to the company. As for Carlos, my guess would be he either joined the BSAA along with Chris and Jill and worked in a different Branch (ex: BSAA South America) or he continued his life and tried to forget about the events of Raccoon City. Billy: He is presumed alive, after the events of Resident Evil 0 he moved to another town/state/country and continued to live his life. Rebecca filled a police report claiming that he died in the Arklay Mountain events (keeping his dog tags as proof), thus ensuring his safety from law enforcement agencies.
I am not claiming these are facts (with the exception of Rebecca), just educated guesses based on the events of the Resident Evil time line. Current whereabouts of Hunk, Carlos, and Billy are unknown and forever shall be unless Capcom would like to fill us all in on the details.
Hope this helped.
-Samuel, Been playing Resident Evil since age 7.