On melodica playing triplet is even easier than the flute. Te-ke-te apparently comes from the Indian konnakol (ta-ki-ta). Selection of suitable combinations of syllables also depends on the native language. For me ,for example, it’s easier to play triplets on pa-ta-ta, that goes 2 times faster than te-ke-to. Can be changed to pa-tu-tu or ku-tu-tu, that changes the timbre a bit.