Recent experiments performed on a variety of soft glassy materials have demonstrated that any imposed shear flow serves to simultaneously fluidize these systems in all spatial directions [Ovarlez , Nature Mater. 9, 115-119 (2010)]. When probed with a second shear flow, the viscous response of the experimental system is determined by the rate of the primary, fluidizing flow. Motivated by these findings, we employ a recently developed schematic mode-coupling theory [Brader , Proc. Natl. Acad. Sci. U.S.A. 106, 15186-15191(2009)] to investigate the three-dimensional flow of a colloidal glass, subject to a combination of simple shear and uniaxial compression. Despite differences in the specific choice of superposed flow, the flow curves obtained show a good qualitative agreement with the experimental findings and recover the observed power-law describing the decay of the scaled viscosity as a function of the dominant rate. We, then, proceed to perform a more formal analysis of our constitutive equation for different kind of "mixed" flows consisting of a dominant primary flow subject to a weaker perturbing flow. Our study provides further evidence that the theory of Brader , Proc. Natl. Acad. Sci. U.S.A. 106, 15186-15191 (2009) reliably describes the dynamic arrest and mechanical fluidization of dense particulate suspensions. (C) 2012 The Society of Rheology. [DOI: 10.1122/1.3676741]