  We describe the resonance fluorescence spectrum of an atomic three-level
system where two of the states are coupled by a single monochromatic laser
field. The influence of the third energy level, which interacts with the two
laser-coupled states only via radiative decays, is studied in detail. For a
suitable choice of parameters, this system gives rise to a very narrow
structure at the laser frequency in the fluorescence spectrum which is not
present in the spectrum of a two-level atom. We find those parameter ranges by
a numerical analysis and use the results to derive analytical expressions for
the additional narrow peak. We also derive an exact expression for the peak
intensity under the assumption that a random telegraph model is applicable to
the system. This model and a simple spring model are then used to describe the
physical origins of the additional peak. Using these results, we explain the
connection between our system, a three-level system in V-configuration where
both transitions are laser driven, and a related experiment which was recently
reported.
