  The sources discovered in deep hard X-ray surveys with 2-8 keV fluxes of
10^-14 erg cm^-2 s^-1 make up the bulk of the X-ray background at these
energies. We present here detailed multi-wavelength observations of three such
sources from the ELAIS Deep X-ray Survey. The observations include sensitive
near-infrared spectroscopy with the Subaru Telescope and X-ray spectral
information from the Chandra X-ray Observatory. The sources observed all have
optical-to-near-IR colours redder than an unobscured quasar and comprise a
reddened quasar, a radio galaxy and an optically-obscured AGN. The reddened
quasar is at a redshift z=2.61 and shows a very large X-ray absorbing column of
N_H approx 3.10^23 cm^-2. This contrasts with the relatively small amount of
dust reddening, implying a gas-to-dust ratio along the line-of-sight a hundred
times greater than that of the Milky Way. The radio galaxy at z=1.57 shows only
narrow emission lines, but has a surprisingly soft X-ray spectrum. The softness
of this spectrum either indicates an unusually low gas-to-dust ratio for the
absorbing medium or X-ray emission related to the young radio source. The host
galaxy is extremely red (R-K=6.4) and its optical/near-IR spectrum is best fit
by a strongly reddened (A_V~2) starburst. The third X-ray source discussed is
also extremely red (R-K=6.1) and lies in a close grouping of three other R-K>6
galaxies. No emission or absorption lines were detected from this object, but
its redshift (and that of one of the nearby galaxies) are constrained by
SED-fitting to be just greater than z=1. The extremely red colours of these two
galaxies can be accounted for by old stellar populations. These observations
illustrate the diverse properties of hard X-ray selected AGN.
