Maharsho says that rashi that says that leveino is 3 by 3 learns like the rivah (tos. d.h. vetoch) that afilu the hanoche is lon something less then 4 by4 if its roieh es hacarca choshuv keminach bemokoim 4x4 the question is that rashi is talking by a levaino thats on the floor so 3x3 still is lovud and it`s botel to the floor so whats the raieh that rashi learns like the rivah that says that din when it`s over 3 (under 10)
The Maharsha understood that since Abaye and Rava explain that the case is dealing with a brick that is three Tefachim high (and that is why one is Patur when the object comes to rest on the top of the brick), then it must be that when we say that he is Chayav when the object comes to rest on the side of the brick, that is also talking about when it is three Tefachim high from the ground, similar to the case of when it rests on its top and he is Patur. This seems to be Rashi's intention as well (in DH Chayav).