  To extend the molecular gas measurements to typical star-forming galaxies
(SFGs) with SFR < 40 Msun yr^{-1} and M* < 2.5x10^{10} Msun at z~1.5-3, we have
observed CO emission with the IRAM Plateau de Bure Interferometer and 30m
telescope for five strongly-lensed galaxies selected from the Herschel Lensing
Survey. These observations are combined with a compilation of CO measurements
from the literature. We infer the luminosity correction factors r2,1 =
0.81+/-0.20 and r3,1 = 0.57+/-0.15 for the J=2 and J=3 CO transitions,
respectively, valid for SFGs at z>1. The combined sample of CO-detected SFGs at
z>1 shows a large spread in star formation efficiency (SFE), such that SFE
extend beyond the low values of local spirals and overlap the distribution of
z>1 sub-mm galaxies. We find that the spread in SFE (or equivalently in
molecular gas depletion timescale) is due to primarily the specific star
formation rate, but also stellar mass and redshift. Correlations of SFE with
the offset from the main-sequence and the compactness of the starburst are less
clear. The increase of the molecular gas depletion timescale with M* now
revealed by low M* SFGs at z>1 and observed at z=0 is in contrast to the
admitted constant molecular gas depletion timescale and the linear
Kennicutt-Schmidt relation. We confirm an increase of the molecular gas
fraction (fgas) from z~0.2 to z~1.2, followed by a very mild increase toward
higher redshifts. At each redshift fgas shows a large dispersion due to the
dependence of fgas on M*, producing a gradient of increasing fgas with
decreasing M*. We provide the first measure of fgas of z>1 SFGs at the low-M*
end (10^{9.4} < M*/Msun < 10^{9.9}), reaching a mean fgas = 0.69+/-0.18, which
shows a clear fgas upturn. Finally, we find evidence for a non-universal
dust-to-gas ratio among high-redshift SFGs and sub-mm galaxies, local spirals
and ULIRGs with near-solar metallicities.
