A novel method is presented to characterise in situ three-phase flow, including wettability, pore occupancy and displacement mechanisms, at the pore scale. We used X-ray microtomography to obtain 3D images of a carbonate reservoir rock saturated with crude oil and formation brine at subsurface conditions. The sample had been aged with crude oil from the same reservoir to replicate the sunsurface wetting conditions. The pore occupancy analysis shows that brine is non-wetting to oil and gas is non-wetting to brine with a wettability order of oil-brine-gas from the most to the least wetting fluid. The waterflood recovery after 1 pore volume injected was only 14%, but this increased to 48% after further gas injection. New multiple displacement mechanisms were observed, with gas displacing brine, which in turn displaces oil. The results from this work can be used to improve the prediction accuracy of the three-phase network models and helps in the design of gas injection processes.