We present reduced-dimensional stochastic projected Gross-Pitaevskii equations describing regimes of confinement and temperature where a one-dimensional (1D) or 2D superfluid is immersed in a 3D thermal cloud. The projection formalism provides both a formally rigorous and physically natural way to effect the dimensional reduction. The 3D form of the number-damping (growth) terms is unchanged by the dimensional reduction. Projection of the energy-damping (scattering) terms leads to modified stochastic equations of motion describing energy exchange with the thermal reservoir. The regime of validity of the dimensional reduction is investigated via variational analysis. For the 1D case, we validate our variational treatment by comparing numerical simulations of a trapped prolate system in three dimensions with the 1D theory and establish a consistent choice of cutoff for the 1D theory. We briefly discuss the scenario involving two components with different degeneracy, suggesting that a wider regime of validity exists for systems in contact with a buffer-gas reservoir.