It needs to have 2 walls and a small portion of a third, if it has Tzurat HaPetach.
The Yerushalmi learns this out from a Limmud of the Pasuk
" Isaiah 4:6:
There will be a sukkah that will serve as a shadow from the heat during the day, a place of refuge, and a cover from storm and from rain.
The Sages maintain that the verse refers to three different activities, and hence require three walls. Rabbi Shimeon counts "a cover from storm and from rain" as two different activities, and hence requires four walls." - http://www.chabad.org/library/article_cdo/aid/946097/jewish/Shofar-Sukkah-vLulav-Chapter-Four.htm
Alternatively, you would say that this is the minimum for an "enclosure" according to Halakah. Usually we assume that what works for Sukkah also works for defining the Reshuyot of Shabbat.