This Brief Communication is concerned with establishing the effect of the presence of a third diffusing component on the onset of convection in a horizontal fluid‐saturated porous layer. The linear stability analysis procedures used in double diffusion are extended to take into account a third diffusing constituent. As a result, general expressions for the various stability boundaries are obtained. The presence of a third diffusing component with small diffusivity can seriously alter the nature of the convective instabilities in the system. For the most part, the findings of the present study for a porous layer exhibit similar behavior to the results reported by Griffiths [J. Fluid Mech.92, 659 (1979)] for triple diffusion in a layer of classical fluid. However, when heat is one of the three diffusing components, the porosity of the porous medium has a considerable effect on the marginal stability boundary for oscillatory convection.