  We report JWST/NIRSpec observations of a star-forming galaxy at $z=2.76$,
MACSJ1149-WR1. We securely detect two temperature-sensitive auroral lines,
[SIII]6312 (7.4$\sigma$) and [OII]7320+7331 doublets (10$\sigma$), and
tentatively [NII]5755 ($2.3\sigma$) for the first time in an individual galaxy
at $z>1$. We perform a detailed analysis of its interstellar media (ISM), and
derive electron temperatures, various heavy element abundances (O/H, N/O, S/O,
and Ar/O) in the hot ionized region, and the neutral fraction in the warm
ionized region. MACSJ1149-WR1 shows a broad feature at the wavelength of HeII
4686, which consists of a broad ($\sim1000$km/s), blue-shifted ($\sim-110$km/s)
line component. Taken together with its mildly elevated N/O abundance, we
conclude that MACSJ1149-WR1 is experiencing a young starburst ($<10$Myr),
likely hosting a large number of Wolf-Rayet stars. None of its spectral
features support the presence of AGN, including: $(i)$ the absence of broad
components and velocity shifts in Hydrogen recombination lines, $(ii)$ low
[FeII]${\rm 1.257 \mu m}$/Pa$\beta$ ratio, and $(iii)$ the absence of
high-ionization lines. Our analysis using HeI lines reveals a higher electron
temperature and a higher attenuation value, indicating that HeI may probe a
smaller spatial scale than HI, presumably the region dominated by the
aforementioned Wolf-Rayet stars. The star formation rates derived from various
HeI lines broadly agree with those from Hydrogen recombination lines. We thus
advocate that HeI can be an excellent, independent probe of multi-phase ISM in
the era of JWST.
