  To understand cosmic mass assembly in the Universe at early epochs, we
primarily rely on measurements of stellar mass and star formation rate of
distant galaxies. In this paper, we present stellar masses and star formation
rates of six high-redshift ($2.8\leq z \leq 5.7$) dusty, star-forming galaxies
(DSFGs) that are strongly gravitationally lensed by foreground galaxies. These
sources were first discovered by the South Pole Telescope (SPT) at millimeter
wavelengths and all have spectroscopic redshifts and robust lens models derived
from ALMA observations. We have conducted follow-up observations, obtaining
multi-wavelength imaging data, using {\it HST}, {\it Spitzer}, {\it Herschel}
and the Atacama Pathfinder EXperiment (APEX). We use the high-resolution {\it
HST}/WFC3 images to disentangle the background source from the foreground lens
in {\it Spitzer}/IRAC data. The detections and upper limits provide important
constraints on the spectral energy distributions (SEDs) for these DSFGs,
yielding stellar masses, IR luminosities, and star formation rates (SFRs). The
SED fits of six SPT sources show that the intrinsic stellar masses span a range
more than one order of magnitude with a median value $\sim$ 5 $\times
10^{10}M_{\Sun}$. The intrinsic IR luminosities range from 4$\times
10^{12}L_{\Sun}$ to 4$\times 10^{13}L_{\Sun}$. They all have prodigious
intrinsic star formation rates of 510 to 4800 $M_{\Sun} {\rm yr}^{-1}$.
Compared to the star-forming main sequence (MS), these six DSFGs have specific
SFRs that all lie above the MS, including two galaxies that are a factor of 10
higher than the MS. Our results suggest that we are witnessing the ongoing
strong starburst events which may be driven by major mergers.
