Antepenult

In linguistics, the antepenult is the antepenultimate syllable of a word (e.g. "-dic-" is the antepenult of "per-pen-dic-ul-ar"). The antepenult precedes the penult or second-to-last syllable which, in turn, precedes the ultimate or final syllable.

In other words, third to last syllable of a word.