  We present results of a large survey of the mid--IR properties of 248 Lyman
Break Galaxies with confirmed spectroscopic redshift using deep Spitzer/IRAC
observations in six cosmological fields. We model the Spectral Energy
Distributions (SEDs) employing a revised version of the Bruzual and Charlot
synthesis population code that incorporates a new treatment of the TP--AGB
phase (CB07). Our primary aim is to investigate the impact of the AGB phase in
the stellar masses of the LBGs, and compare our new results with previous
stellar mass estimates. Based on the new CB07 code we find that the stellar
masses of LBGs are smaller on average by a factor of ~1.4 compared to previous
estimates. LBGs with 8um and/or 24um detections show higher masses (M~10^11 Mo)
than LBGs faint in the IRAC bands (M~10^9 Mo). The ages of these massive LBGs
are considerably higher than the rest of the population, indicating that they
have been star-forming for at least ~1 Gyr. We also show how the addition of
the IRAC bands, improves the accuracy of the estimated stellar masses and
reduced the scatter on the derived M/L ratios. In particular, we present a
tight correlation between the 8um IRAC band (rest-frame K for galaxies at z~3)
and the stellar mass. We calculate the number density of massive (M > 10^11 Mo)
LBGs and find it to be $\Phi$= (1.12 $\pm$ 0.4) x 10^(-5) Mpc^(-3), ~1.5 times
lower than that found by previous studies. Finally, based on UV-corrected SFRs
we investigate the SFR-stellar mass correlation at z~3, find it similar to the
one observed at other redshifts and show that our data place the peak of the
evolution of the specific star formation rate at z~3.
