Several words fit your ?R?C?? but unfortunately, none of them means 'letters'
ARCCOS
ARECAS
BRACED
BRACER
BRACES
BRACHS
BRACKS
BRACTS
BRECHT
BRICKS
BRICKY
BROCHE
BROCHS
BROCKS
BRUCIN
BRUCKE
CRACKS
CRACKY
CRACOW
CRECHE
CRICKS
CRICKY
CROCHE
CROCKS
CROCUS
CRUCES
CRUCKS
DRACHM
DRECKS
DRECKY
DRICES
ERECTS
ERICAS
ERICKS
ERUCTS
FRACAS
FRACTI
FRACTS
FRICHT
FROCKS
GRACED
GRACES
GRACIE
GRECES
GRICER
GRICES
GROCER
GRYCES
ORACHE
ORACLE
PRECIS
PRECUT
PRICED
PRICER
PRICES
PRICEY
PRICKS
PRICKY
TRACED
TRACER
TRACES
TRACEY
TRACKS
TRACTS
TRECKS
TRICAR
TRICED
TRICES
TRICIA
TRICKS
TRICKY
TRICOT
TROCAR
TROCHE
TROCHI
TROCKS
TRUCED
TRUCES
TRUCKS
URACHI
URACIL
WRACKS
WRECKS
WRICKS