May a sliding or removable door count as one of the three Halachically required walls?[1]
Yes, a sliding door may be used as one of the three minimum required walls of a Sukkah, however the Sukkah is only Kosher when the door is closed, or if a wall of 57.2 centimeters [or of Lavud] remains within that side of the Sukkah even when the door is open. [Thus, if the other side of the sliding door is always closed, and is also part of the Sukkah, within the space of the Sechach, then the door is valid as a wall, as one only requires 57 centimeters for a valid wall.] If the other side of the sliding door is not part of the Sukkah, then the Sukkah is only Kosher when the door is closed, and when the door is opened, the Sukkah is considered to have lost its wall and is considered invalid. Furthermore, at the initial time that the Sechach is erected, the doors must be closed. However, once erected properly, it is permitted later on to open and close this door, even though it nullifies the Sukkah at the time of its opening. One must note that on Shabbos and Yom Tov it would remain forbidden to open and close this door, being it is invalidated when opened, and then re-validated when closed.[2] See Halacha 8!
[1] Mikraeiy Kodesh Sukkos 1:110-11; Shevet Halevi 7:56 and 8:146; Shraga Hameir 7:137; Orchos Rabbeinu 2:219; SSH”K 25 footnote 115; Piskeiy Teshuvos 630:3; 635:2; See however Kinyan Torah 3:93 who argues that the Sukkah remains Kosher even when the doors are opened; Koveitz Mibeis Levi 2:23; 3:109 and 162
[2] See Poskim ibid and Piskeiy Teshuvos 630:3