Not really. Three-vowel systems are pretty common: they’re also found in Classical Arabic, Quechua, most Australian and Eskimo–Aleut languages, and a bunch of o

Not really. Three-vowel systems are pretty common: they’re also found in Classical Arabic, Quechua, most Australian and Eskimo–Aleut languages, and a bunch of others. Also, saying that Old Person had ‘three vowels’ while Sanskrit had ‘sixteen vowels’ is comparing apples to oranges a little bit. It seems like Old Persian had three basic vowel qualities, each of which could be short or long; meanwhile, Sanskrit had five basic vowel qualities, each of which could be short or long, as well as syllabic consonants /ṛ ḷ/ which could also be short or long, as well as /ṃ ḥ/ after vowels. It’s bigger, to be sure, but Sanskrit only two more basic vowel qualities than Old Persian (or four if you count the syllabic consonants).