Well... 8 12s would need a little less air space, by about 2 cubes or so, and have more cone area. However it could be cheaper to get 3 BTLs than 8 Chaos, but I'm not sure as I don't know AA pricing. As far as motor force to cone area, it probably comes out pretty close.
Here's one thing to consider: If you order the three BTLs with D1.4 ohm coils you can wire the subs to .93 ohms, and to 2.1 ohms, meaning that you can safely use 1 amp wired to about 1 ohm, or strap two amps at 2 ohms without having to modify anything except for wiring. That's something you can't do with the 8 12s.