  We present the third Fermi Large Area Telescope source catalog (3FGL) of
sources in the 100 MeV-300 GeV range. Based on the first four years of science
data from the Fermi Gamma-ray Space Telescope mission, it is the deepest yet in
this energy range. Relative to the 2FGL catalog, the 3FGL catalog incorporates
twice as much data as well as a number of analysis improvements, including
improved calibrations at the event reconstruction level, an updated model for
Galactic diffuse gamma-ray emission, a refined procedure for source detection,
and improved methods for associating LAT sources with potential counterparts at
other wavelengths. The 3FGL catalog includes 3033 sources above 4 sigma
significance, with source location regions, spectral properties, and monthly
light curves for each. Of these, 78 are flagged as potentially being due to
imperfections in the model for Galactic diffuse emission. Twenty-five sources
are modeled explicitly as spatially extended, and overall 232 sources are
considered as identified based on angular extent or correlated variability
(periodic or otherwise) observed at other wavelengths. For 1009 sources we have
not found plausible counterparts at other wavelengths. More than 1100 of the
identified or associated sources are active galaxies of the blazar class;
several other classes of non-blazar active galaxies are also represented in the
3FGL. Pulsars represent the largest Galactic source class. From source counts
of Galactic sources we estimate the contribution of unresolved sources to the
Galactic diffuse emission is ~3% at 1 GeV.
