  We present millimetre dust emission measurements of two Lyman Break Galaxies
at z~3 and construct for the first time fully sampled infrared spectral energy
distributions (SEDs), from mid-IR to the Rayleigh-Jeans tail, of individually
detected, unlensed, UV-selected, main sequence (MS) galaxies at $z=3$. The SED
modelling of the two sources confirms previous findings, based on stacked
ensembles, of an increasing mean radiation field <U> with redshift, consistent
with a rapidly decreasing gas metallicity in z > 2 galaxies. Complementing our
study with CO[3-2] emission line observations, we measure the molecular gas
mass (M_H2) reservoir of the systems using three independent approaches: 1) CO
line observations, 2) the dust to gas mass ratio vs metallicity relation and 3)
a single band, dust emission flux on the Rayleigh-Jeans side of the SED. All
techniques return consistent M_H2 estimates within a factor of ~2 or less,
yielding gas depletion time-scales (tau_dep ~ 0.35 Gyrs) and gas-to-stellar
mass ratios (M_H2/M* ~ 0.5-1) for our z~3 massive MS galaxies. The overall
properties of our galaxies are consistent with trends and relations established
at lower redshifts, extending the apparent uniformity of star-forming galaxies
over the last 11.5 billion years.
