Kayla Freilich
Why the middle one?
Many people know that the reason that we break one of the Matzot on Pesach is because it is referred to in the Torah as “Lechem Oni”, “bread of poverty,” and the Gemara teaches us that that means that we must use a broken piece of Matzah. The breaking symbolizes the splitting of the Red Sea and that is the reason that the breaking must be done at the table as part of the Seder. However all of this would still apply if we broke the top or bottom Matzah, why the middle one specifically? Wouldn’t it make more sense to break the top one because of the principle that “one should not pass over a Mitzvah article (with his hand)?” And by passing over the top Matzah you are in turn going against this principle. The Bach explains that the reason that we do not break the top Matzah is because that would present problems later on in the Seder, when the Brachot are made over the Matzot (as there is no Bracha made at Yachatz). There are three Matzot on Pesach because there are three Mitzvot regarding the Matzot. You need two full pieces to fulfill the Mitzvah of Lechem Mishna and you need the broken one to fulfill the idea of Lechem Oni. Thus, when the Bracha of Hamotzei is made the two full ones are held, when the Bracha of “Al Achilat Matzah” is made the broken one is held and after pieces of both of these Matzot have been eaten, the remaining Matzah is used for the Mitzvah of Korech. Each piece of Matzah has their own unique purpose; they are each used for a different Mitzvah. The top piece of Matzah is used for the first Mitzvah (Hamotzei), the middle piece is used for the second Mitzvah (Al Achilat Matzah), and the bottom piece is used for the third Mitzvah (Korech). So now if the top Matzah had been broken it would have to be skipped over to fulfill the first Mitzvah (Hamotzei), thereby having us violate the principle of “one should not pass over a Mitzvah article (with his hand).” This is the reason that the Bach says that the middle Matzah is specifically the one broken.
Sources:
The Hegyonei Halacha Haggadah by Rabbi Yitzchak Mirsky