Bill,
While the 3 foot wide hall/doorway clearance would allow passage of the wheelchair either powered or pushed, when operated manually by the user, at least 4 foot clearance is needed to allow smooth travel without elbow damage.
My preference for "U" shape is based on observing that most ranges I've shot at use bench spacing of 10 foot or less to maximize the number of available positions (we use 8 foot spacing at my local range). At another local range, they have very nice permanent concrete modified "T" tops at 10 foot centers. We tested that arrangement and found inadequate clearance if there was a right-left or left-right pair of wheelchair shooters or one wheelchair and one stool shooter during entry or departure without careful movement coordination.
Note that all the wheelchair bound shooters that I have worked with are very dedicated and amazingly tolerant of poorly designed accommodations, the rest of us could provide them with a far more enjoyable experience if we put just a little more effort into our range designs. There is a locally popular Indian phrase that loosely translates to "first walk a mile in his moccasins" to think about when dealing with the abilities of others.
Fred Bohl