Forget material substrata for a moment. Instead think of matter as a lowest level of form (ala Polanyi). Doesn't that make the contingency of material things all the more obvious... and hence pave the way for a version of the third way that would work? Weird question: what if one's starting point were the existence of a multiplicity of angelic forms? Would the very fact that these immaterial forms are different underscore the fact that each one of them is Contingent? From which would follow yet another workable version of the third way. Perhaps.