2022A&A...663A..22G

High-redshift dusty star-forming galaxies with very high star formation rates (500-3000 M_☉ yr^–1) are key to understanding the formation of the most extreme galaxies in the early Universe. Characterising the gas reservoir of these systems can reveal the driving factor behind the high star formation. Using molecular gas tracers such as, high-J CO lines, neutral carbon lines, and the dust continuum, we can estimate the gas density and radiation field intensity in their interstellar media. In this paper, we present high resolution (∼0.4'') observations of CO(7-6), [CI](2-1), and dust continuum of three lensed galaxies from the South pole telescope - sub-millimetre galaxies (SPT-SMG) sample at z ∼ 3 with the Atacama Large Millimetre/submillimetre Array. Our sources have high intrinsic star formation rates (> 850 M_☉ yr^–1) and rather short depletion timescales (< 100 Myr). Based on the L_[CI](2–1)/L_CO(7 - 6)_ and L_[CI](2–1)/L_IR ratios, our galaxy sample has similar radiation field intensities and gas densities compared to other submillimetre galaxies. We performed visibility-based lens modelling on these objects to reconstruct the kinematics in the source plane. We find that the cold gas masses of the sources are compatible with simple dynamical mass estimates using ULIRG-like values of the CO-H₂ conversion factor α_CO, but not Milky Way-like values. We find diverse source kinematics in our sample: SPT0103-45 and SPT2147-50 are likely rotating disks, while SPT2357-51 is possibly a major merger. The analysis presented in the paper could be extended to a larger sample to determine better statistics of morphologies and interstellar medium properties of high-z dusty star-forming galaxies.