Question: Does one need lechem mishne for seuda shelishis? If so, can I use a frozen challa? Do I need to remove it from the bag if I’m not going to eat it?
Answer: The Gemara (Shabbos 117b) writes that we are botzeia over two challos to remember that two portions of man fell in the midbar before Shabbos. While according to the Magen Avraham (OC 254:23) this requirement is derabanan, the Taz (OC 678:2), Chasam Sofer (OC 46) and Aruch Hashulchan (OC 274:1) write that it is mideoraisa.
Following Rashi, the Shulchan Aruch (OC 274:1) understands this to mean saying hamotzi over the two challos. The Vilna Gaon, however, follows Rashba who writes that the requirement is to cut both challos (See Mishna Berura 274:4; Aruch Hashulchan OC 274:3).
The Daas Zekeinim (Shemos 16:22) writes that as the bnei yisrael would only have had one portion of man left by Shabbos afternoon, there is no need to have lechem mishne for seuda shelishis. The Rema (OC 291:4) writes that while many only use one challa for seuda shelishis it is ideal to use two challos (See Shemiras Shabbos Kehilchasa 55:2).
R’ Ovadia Yosef (Yabia Omer 8 OC:32), R’ Eliezer Waldenberg (Tzitz Eliezer 11:23) and R’ Moshe Sternbuch (Teshuvos Vehanhagos 2:170) write that ideally one should only use fresh challa, though R’ Ephraim Greenblatt (Rivevos Ephraim 2:115:2) wrote that he often used a frozen challa for lechem mishne as by the end of the meal, the challa had defrosted and was perfectly edible (See Minchas Yitzchak 9:42; Shemiras Shabbos Kehilchasa 55:n39).R’ Ephraim Greenblatt (Rivevos Ephraim 1:201) and R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 55:n38) write that ideally, the second challa should be removed from the bag before saying hamotzi.