Method of operating a three-phase slurry reactor includes feeding at a low level at least one gaseous reactant into a vertically extending slurry body of solid particles suspended in a suspension liquid, the slurry body being contained in at least two vertically extending shafts housed within a common reactor shell, each shaft being divided into a plurality of vertically extending channels at least some of which are in slurry flow communication and the slurry body being present in at least some of the channels. The gaseous reactant is allowed to react as it passes upwardly through the slurry body present in at least some of the channels of the shafts, thereby to form a non-gaseous and/or a gaseous product. Gaseous product, if present, and/or unreacted gaseous reactant is allowed to disengage from the slurry body in a head space above the slurry body.