antepenultimate

Short definition from Wiktionary

Adjective

1. 1
   Two before the last, i.e., the one immediately before the penultimate, in a series.

Noun

1. 1
   The third before the last in a series. e.g. (..., antepenultimate, penultimate, ultimate)
2. 2
   The syllable that comes two before the last in a word.