No word on western release
It’s official: Seiken Densetsu 3 will be on the Nintendo Switch, included in the Seiken Densetsu Collection along with Seiken Densetsu 1 (Final Fantasy Adventure) and Seiken Densetsu 2 (Secret of Mana). The collection will be 4,800 yen in Japan (~$43). There is no word yet on a localization, but since the Switch is region-free you could play it if you can read Japanese or use a translation guide.
For a pack of three ancient games without any significant visual upgrades, the price is a bit much, but for those waiting for the third game this package perhaps provides some hope. The most I can do is report on it.