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 ($2.8leq z leq 5.7$) 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.
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