  We present an analysis of the X-ray emission of a large sample of z~3 Lyman
break galaxies (LBGs), based on Chandra/ACIS observations of several LBG survey
fields. 24 LBGs are directly detected in the X-ray. 13 of the LBGs have optical
spectroscopic signatures of AGN activity, but almost all the other X-ray
detections are also likely to host an accreting black hole based on their X-ray
properties. The AGN exhibit a wide range in X-ray luminosity, from weak
Seyferts to bright QSOs. Optical spectroscopy identified approximately 1/3 of
the X-ray detected sources as broad line QSOs, 1/3 as narrow line AGN and 1/3
as normal star forming LBGs. The fraction of X-ray detected LBGs is 3%, much
lower than has been found for submm selected galaxies. 2 galaxies have X-ray
luminosities, spectra and f_x/f_opt values that are consistent with emission
from star formation processes and are candidate X-ray bright, pure starburst
galaxies at z~3. X-ray spectral analysis of the LBGs shows a mean photon index
1.96, similar to local AGN. There is evidence for absorption in at least 40% of
the objects. Significantly more absorption is evident in the narrow line AGN,
consistent with AGN unification schemes. After correction for absorption the
narrow and broad line objects show the same average luminosity. LBGs
spectroscopically classified as normal galaxies are less luminous in X-ray
indicating that the host galaxy is outshining any optical AGN signature. For
the LBGs without direct detections, stacking the X-ray flux in the 2 deepest
fields under consideration (the HDF-N and GWS) produced significant detections
in each. The detection in the HDF-N gives an X-ray derived SFR of 42.4+/-7.8
Msolar/yr per LBG and, by comparing with the UV SFR, the implied UV extinction
correction is 4.1+/-0.8. (abridged)
