In this paper, based on first principles calculations, we propose functionalization of group-III monochalcogenides with halogen atoms to form Janus MXY (M = Al, Ga, In; X = Se, Te; Y = Br, I) monolayers with enhanced physical properties. Using phonon dispersion, formation energy and AIMD simulations, nine MXY monolayers are found to be highly stable. The proposed MXY structures are shown to be direct/indirect semiconductors with bandgaps in the range of 1.13 and 3.34 eV. With regard to SOC effect, large Rashba spin splitting is observed at the Γ-point of conduction band in all Janus MXY monolayers with the largest coefficient of 1.021 eVÅ for GaTeBr. Moreover, we apply vertical electric field and biaxial strain as methods to tune the electronic and spintronic properties of MXY monolayers and improve their potential applications in the future devices. Investigating band edge positions, we find that six MXY monolayers fulfil the band alignment requirement for overall water splitting. Finally, Optical absorption, spatial charge separation and Gibbs free energy profile as other factors to assess the efficiency of MXY photocatalysts are studied. We believe that our research can pave a solid way to tune the physical properties of group-III monochalcogenides for a wider range of applications.