In the Diaspora, one can have a situation when one needs Eruv Tavshilin three times within a short period of just three weeks, namely when Rosh Hashanah, Succos and Shemini Atzeres occur on Thursday and Friday. The general custom is to make it on the day before each of these Festivals. One might well ask whether one can make Eruv Tavshilin just once before Rosh Hashanah, then preserve the tavshilin, for example in the freezer? The poskim are divided on this question. There is also a third intermediate opinion, which allows one to make Eruv Tavshilin before Succos to also include Shemini Atzeres, the reason being that they are regarded as the same Festival. (Shulchan Aruch Orach Chaim 527:14 and Mishnah Berurah (including Biur Halachah) on it)