  Atomic Carbon can be an efficient tracer of the molecular gas mass, and when
combined to the detection of high-J and low-J CO lines it yields also a
sensitive probe of the power sources in the molecular gas of high redshift
galaxies. The recently installed SEPIA5 receiver at the focus of the APEX
telescope has opened up a new window at frequencies 159 - 211 GHz allowing the
exploration of the Atomic Carbon in high-z galaxies, at previously inaccessible
frequencies from the ground. We have targeted three gravitationally lensed
galaxies at redshift of about 3 and conducted a comparative study of the
observed high-J CO/CI ~ratios with well-studied nearby galaxies. Atomic Carbon
(CI(2-1)) was detected in one of the three targets and marginally in a second,
while in all three targets the $J=7\to6$ CO line is detected. The
CO(7-6)/CI(2-1), CO(7-6)/CO(1-0) line ratios and the CO(7-6)/(far-IR continuum)
luminosity ratio are compared to those of nearby objects. A large excitation
status in the ISM of these high-z objects is seen, unless differential lensing
unevenly boosts the CO line fluxes from the warm and dense gas more than the
CO(1-0), CI(2-1), tracing a more widely distributed cold gas phase. We provide
estimates of total molecular gas masses derived from the atomic Carbon and the
Carbon monoxide CO(1-0), which within the uncertainties turn out to be equal.
