  We investigate the properties of 1088 Lyman Break Galaxies (LBGs) at z~3
selected from a ~2.63$ deg2 sub-region of the First Look Survey field using the
ground-based multi-color data and the Spitzer Space Telescope mid-infrared data
at 3--8 and 24 um. With the wide area and the broad wavelength coverage, we
sample a large number of ``rare'' u-band dropouts which are massive (M* > 10^11
Msun), allowing us to perform a statistical analysis of these subsets of LBGs
that have not been studied in detail. Optically bright (R(AB) < 24.5 mag) LBGs
detected in mid-infrared (S_{3.6um} > 6 uJy) reside at the most massive and
dusty end of the LBG population, with relatively high and tight $M/L$ in
rest-frame near-infrared. Most infrared-luminous LBGs (S_{24um} > 100 uJy) are
dusty star-forming galaxies with star formation rates of 100--1000 Msun/yr,
total infrared luminosity of > 10^12 Lsun. By constructing the UV luminosity
function of massive LBGs, we estimate that the lower limit for the star
formation rate density from LBGs more massive than 10^11 Msun at z~3 is > 3.3 x
10^-3 Msun/yr/Mpc^3, showing for the first time that the UV-bright population
of massive galaxies alone contributes significantly to the global star
formation rate density at z~3. When combined with the star formation rate
densities at z < 2, our result reveals a steady increase in the contribution of
massive galaxies to the global star formation from z=0 to z=3, providing strong
support to the downsizing of galaxy formation.
