Just for amusement, I put "Triple Crown" in the Kentuckiana DRF searchable database. There were a lot of hits before 1930 generated, most of them dealing with the British TC (coverage of British racing was fairly decent in the big American dailies even just after the Civil War, given the limitations of the times). The idea of applying British terminology to the American scene was hardly new, since Derbies and Oaks were common around the country by the time the '30s rolled around.
Anyway, I found several articles that proposed applying "TC" to various combinations of American races. Here is one from 1923 that is very concrete regarding the present modern combination, although in small case letters. The author presumed a consensus that the Preakness, KD, and Belmont were THE "big three" classics at that point in history. Personally, I think it pushes back the official story a little further back in time.
http://kdl.kyvl.org/cgi/t/text/pageview ... assterms=1
I don't have the distraction time available to keep on the DRF hunt. Here's the starting point for more DRF search engine fun:
http://kdl.kyvl.org/drf/