GO-NAD!, I'm not one to have suggestions here, but methinks that if you list some parameters, it might help in that others can narrow down that huge list of possibilities as far as DIY builds. So, on the behalf of everyone:
1. What is the budget for three speakers all said and done?
2. What are the ideal dimensions, or more particularly the limitations?
I was thinking about something, but it will take me a bit to get to . . .
3a. Your ceiling is 7' high?
3b. You are still building the equipment rack below the screen, and if so how tall will this be?
3c. Ideal tweeter height would be about say 40" high?
The reason I was curious about the second set of questions was to maybe have an upside down tower configuration, depending on the height of the rack. I know it would be very strange, but then you can start getting access to more capable midbass, as I've already offered that I think that could be the hardest thing for speakers when movies are concerned.
Towers in an upright configuration on top of a cabinet that is say more than a couple of feet tall might start getting too high.
Otherwise, I see bookshelves on short* but sturdy (perhaps sand filled) stands that are sitting on top of the cabinet.
Or, you drill into the speakers so that you can use beefier speaker mounts into bookshelves, coming forward from the front wall. You may very well have to concoct/fabricate arms or something that are strong enough. Or, you have a shallower depth to the false/screen wall.
http://www.htd.com/speaker-cables-accessories/Wall-Mount-Speaker-Brackets
I know rmk uses these mounts for use with his industrial grade JTR speakers, and I believe he drilled right into them as well. He was wondering about it to the designer, and he responded with, what's the big deal do it. If I did it all over again, I woulda kept my B25s and drilled into them, instead of downgrading to Alphas only for the sake of ceiling mounting without drilling.