Why a 3:30 Mincha at BTYA?
On a regular erev Yom Tov (and even erev Shabbos), one should not eat a seudah of any size from the tenth hour of the day (these days 4:10 pm). If erev Yom Tov is Shabbos, and one did not yet eat seudah shlishis by this time (although he should have), he may eat it after this time, but he must minimize what he eats so that it is less than what would be considered a normal seudah.
Al pi kabbalah, one should eat seudah shlishis on Shabbos only after one has davened Minchah.
THEREFORE, the Shul, in trying to accommodate those who want to do things in the ideal way (but can’t, for whatever reason, make it to the regular 1:20 pm minchah on Shabbos), will have a 3:30pm minchah. There will also be another mincha at 7:15pm.
Candle-lighting time on the night of Yom Tov is not before 8:30 pm, and “baruch hamavdil bein kodesh l’kodesh” must be said before doing this or any melachah.