  We detect bright emission in the far infrared fine structure [O III] 88$\mu$m
line from a strong lensing candidate galaxy, H-ATLAS J113526.3-014605,
hereafter G12v2.43, at z=3.127, using the $\rm 2^{nd}$ generation Redshift (z)
and Early Universe Spectrometer (ZEUS-2) at the Atacama Pathfinder Experiment
Telescope (APEX). This is only the fifth detection of this far-IR line from a
sub-millimeter galaxy at the epoch of galaxy assembly. The observed [O III]
luminosity of $7.1\times10^{9}\,\rm(\frac{10}{\mu})\,\rm{L_{\odot}}\,$ likely
arises from HII regions around massive stars, and the amount of Lyman continuum
photons required to support the ionization indicate the presence of
$(1.2-5.2)\times10^{6}\,\rm(\frac{10}{\mu})$ equivalent O5.5 or higher stars;
where $\mu$ would be the lensing magnification factor. The observed line
luminosity also requires a minimum mass of $\sim 2\times
10^{8}\,\rm(\frac{10}{\mu})\,\rm{M_{\odot}}\,$ in ionized gas, that is $0.33\%$
of the estimated total molecular gas mass of
$6\times10^{10}\,\rm(\frac{10}{\mu})\,\rm{M_{\odot}}\,$. We compile multi-band
photometry tracing rest-frame UV to millimeter continuum emission to further
constrain the properties of this dusty high redshift star-forming galaxy. Via
SED modeling we find G12v2.43 is forming stars at a rate of 916
$\rm(\frac{10}{\mu})\,\rm{M_{\odot}}\,\rm{yr^{-1}}$ and already has a stellar
mass of $8\times 10^{10}\,\rm(\frac{10}{\mu})\,\rm{M_{\odot}}\,$. We also
constrain the age of the current starburst to be $\leqslant$ 5 million years,
making G12v2.43 a gas rich galaxy lying above the star-forming main sequence at
z$\sim$3, undergoing a growth spurt and, could be on the main sequence within
the derived gas depletion timescale of $\sim$66 million years.
