  We present results from Atacama Large Millimeter/submillimeter Array (ALMA)
spectral line-scan observations at 3-mm and 2-mm bands of three
near-infrared-dark (NIR-dark) galaxies behind two massive lensing clusters MACS
J0417.5-1154 and RXC J0032.1+1808. Each of these three sources is a faint
(de-lensed $S_{\text{1.2 mm}}$ $<$ 1 mJy) triply lensed system originally
discovered in the ALMA Lensing Cluster Survey. We have successfully detected CO
and [C I] emission lines and confirmed that their spectroscopic redshifts are
$z=3.652$, 2.391, and 2.985. By utilizing a rich multi-wavelength data set, we
find that the NIR-dark galaxies are located on the star formation main sequence
in the intrinsic stellar mass range of log ($M_*$/$M_\odot$) = 9.8 - 10.4,
which is about one order of magnitude lower than that of typical submillimeter
galaxies (SMGs). These NIR-dark galaxies show a variety in gas depletion times
and spatial extent of dust emission. One of the three is a normal star-forming
galaxy with gas depletion time consistent with a scaling relation, and its
infrared surface brightness is an order of magnitude smaller than that of
typical SMGs. Since this galaxy has an elongated axis ratio of $\sim 0.17$, we
argue that normal star-forming galaxies in an edge-on configuration can be
heavily dust-obscured. This implies that existing deep WFC3/F160W surveys may
miss a fraction of typical star-forming main-sequence galaxies due to their
edge-on orientation.
