  We present CO(J= 1-0; 3-2; 5-4; 10-9) and 1.2-kpc resolution [CII] 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 hyper-luminous infrared galaxy
(LIR=(4+/-1)x10^13 Lsun) with a total molecular gas mass of (2.1+/-0.7)x10^11
(alpha_CO/0.8) Msun. The CO(J=1-0) and [CII] emission are extended over ~9 kpc
in diameter, and the CO line FWHM exceeds 1100 km s^-1. The [CII] emission
shows a monotonic velocity gradient consistent with a disk, with a maximum
rotation velocity of v_c = 616+/-100 km s^-1 and a dynamical mass of
(7.7+/-3.1)x10^11 Msun. We find a star formation rate (SFR) of 2900^750_-595
Msun yr^-1. HXMM05 is thus among the most intensely star-forming galaxies known
at high redshift. Photo-dissociation region modeling suggests physical
conditions similar to nearby SFGs, showing extended star formation, which is
consistent with our finding that the gas and dust emission are co-spatial. Its
molecular gas excitation resembles the local major merger Arp 220. The broad CO
and [CII] 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.
