  We describe the Dark Energy Survey (DES) Deep Fields, a set of images and
associated multi-wavelength catalogue ($ugrizJHKs$) built from Dark Energy
Camera (DECam) and Visible and Infrared Survey Telescope for Astronomy (VISTA)
data. The DES Deep Fields comprise 11 fields (10 DES supernova fields plus
COSMOS), with a total area of $\sim30~$ square degrees in $ugriz$ bands and
reaching a maximum $i$-band depth of 26.75 (AB, $10\sigma$, 2 arcsec). We
present a catalogue for the DES 3-year cosmology analysis of those four fields
with full 8-band coverage, totalling $5.88~$ sq. deg. after masking. Numbering
$2.8~$million objects ($1.6~$million post masking), our catalogue is drawn from
images coadded to consistent depths of $r=25.7, i=25, z=24.3$ mag. We use a new
model-fitting code, built upon established methods, to deblend sources and
ensure consistent colours across the $u$-band to $Ks$-band wavelength range. We
further detail the tight control we maintain over the point-spread function
modelling required for the model fitting, astrometry and consistency of
photometry between the four fields. The catalogue allows us to perform a
careful star-galaxy separation and produces excellent photometric redshift
performance (${\rm NMAD} = 0.023$ at $i<23$). The Deep-Fields catalogue will be
made available as part of the cosmology data products release, following the
completion of the DES 3-year weak lensing and galaxy clustering cosmology work.
