The gemara says that R Yehuda according to his reasoning a suka needs kvah Where do we see this? I know he says that a suka can be over 20 amos. but he agrees that a suka can be less, Does he not say a suka can be built with only 3 walls So he agrees to temporary too?
See our INSIGHTS to Daf 23a:2, in which we suggest that according to Rashi, Rebbi Yehudah's source is the verse quoted there (which requires that a Sukah be built strong enough to stand for seven days in one place, implying at least some sort of permanence).
A Sukah with three walls is valid since it is fit to stand for an extended period. A mobile Sukah, though, is not considered fixed since it does not stay in one place for seven days, and therefore it is not considered at all "permanent." Rebbi Yehudah invalidates such a Sukah.
Other Rishonim clearly do not learn that the source is the verse we have mentioned above. If so, Rebbi Yehudah apparently applies the principle of "Teshvu k'Ein Taduru" to mean that just as one lives in a permanent house during the year, so, too, his Sukah must have an element of permanence to it. Again, though, the exact element of permanence is subject to dispute among the Tana'im, as we find on Daf 7b, and a Sukah with three walls is certainly considered to be permanent.