A rhetorical figure, a building series of 3 phrases or cola in immediate succession, each having the same grammatical function within the sentence. It was used quite commonly in ancient oratory, and was thought to gradually intensify the impact of the series, with strong emphasis on the final element.
Thus, Cicero in his Pro Caelio begins: "Si quis, iudices, forte nun adsit ignarus legum iudiciorum consuetudinisque nostrae...", "If anyone, gentlemen of the jury, should perchance be present who is ignorant of our laws, or or trials, and our custom..". The sequence develops from the written statues (legum) to the execution of criminal trials (iudiciorum) to social habit and moral expectations (consuetudinis), having little at all to do with formal law.
It was suggested above that a tricolon crescens introduces an emphatic element (the first refuge of every wicked philologist). Often, this is true; there is a definite build from the initial to the final element of the series. Just as often, however, this is utter poppycock. A similar phrase, tricolon decrescens, or "descending tricolon" has come into vogue to describe a tapering series, though this is quite senseless; as in the above passage, it is argued that, while indeed the progression focuses on an ever more narrow or personal moral guideline, the focus builds to a final element by its very nature. In any case, the phrase describes a particular ordering, regardless of function, in ancient and modern literature.