With that said ... it's not like I have not thought about doing what you are suggesting.
In one of my systems, I was placing the speakers on their sides, on bookcase shelves, and the bookcase had a back wall.
The speakers looked like ... [ o O ] [ O o ].
It never worked great, at its best it was OK to very good. The sealed back on the bookcase posed the biggest challenge, as I was placing a box inside of a box. That and keeping my cats from scratching or toppling my speakers.
The two speakers that did work best were ... Omega single driver speakers and Ascend Acoustic speakers because the Ascends are designed to work either vertically or horizontally. I tried this with Totem Mites and the results were not good.
The solution for me was purchasing the Ascend Sierras, placing them horizontally, on top of the bookcases ... a big improvement, but not perfect.
Many people here use two excellent L/R speakers and do not bother with a center channel ... a 2 channel HT system.
At this point, why are you considering doing this? Other than space considerations, is something else wrong that you are looking to correct?