In the process of working on a chapter of my e-book covering interior composition I came across a couple of shots of a little condo we sold last year that nicely illustrates the pros and cons of a three wall shot compared to a two wall shot.
The upper shot on the right, is effectively a two wall shot and the bottom photo is a three wall shot of the same room. The two wall shot was one of the photos we used to market this home. I liked it the best of the 4 or 5 shots I took of this little living room and the stager, the agent, and the home owner all agreed with my choice. At the time I didn't think about why we all liked the two wall shot the best.
After seeing a discussion in the PFRE flickr group on this subject I realized why the two wall shot feels more spacious and is visually more pleasing.
As Jeremy Esland explains in the PFRE flickr discussion on this subject, "as photographers, as long as we are aware of the potential effects, we are, of course, free to ignore them, compensate for them or even make use of them."
So you can use a three wall shot to illustrate how large a room is but if you are not careful there can be some down sides of showing three walls.