I do not agree with a given "Triphone" definition. According to Jurafsky (D. Jurafsky, J. H. Martin, Speech And Laguage Processing, p. 380, Pearson / Prentice Hall 2009) triphone is an abstract object which we use to describe or name an acoustic model of a phoneme in some particular phonetic context. It is not a concatenation of 3 following phonemes. Similar description was given by L. Rabiner and E. Young as well.

Jakub Galka