We present CO (J = 1 → 0; 3 → 2; 5 → 4; 10 → 9) and 1.2 kpc resolution [C II] line observations of the dusty
star-forming galaxy (SFG) HXMM05—carried out with the Karl G. Jansky Very Large Array, the Combined
Array for Research in Millimeter-wave Astronomy, the Plateau de Bure Interferometer, and the Atacama Large
Millimeter/submillimeter Array, measuring an unambiguous redshift of z = 2.9850 ± 0.0009. We find that
HXMM05 is a hyperluminous infrared galaxy (LIR = (4 ± 1) × 1013 Le) with a total molecular gas mass of
(2.1 ± 0.7) × 1011(aCO/0.8) Me. The CO (J = 1 → 0) and [C II] emission are extended over ∼9 kpc in diameter,
and the CO line FWHM exceeds 1100 km s−1
. The [C II] emission shows a monotonic velocity gradient consistent
with a disk, with a maximum rotation velocity of vc = 616 ± 100 km s−1 and a dynamical mass of
(7.7 ± 3.1) × 1011 Me. We find a star formation rate of 2900-
+
595
750 Me yr−1
. HXMM05 is thus among the most
intensely SFGs known at high redshift. Photodissociation region modeling suggests physical conditions similar to
nearby SFGs, showing extended star formation, which is consistent with our finding that the gas emission and dust
emission are cospatial. Its molecular gas excitation resembles the local major merger Arp 220. The broad CO and
[C II] lines and a pair of compact dust nuclei suggest the presence of a late-stage major merger at the center of the
extended disk, again reminiscent of Arp 220. The observed gas kinematics and conditions, together with the
presence of a companion and the pair of nuclei, suggest that HXMM05 is experiencing multiple mergers as a part
of the evolution.