Can I use a 3x140 possibly for all my hardware and mount it on top?
Perhaps, but you'll see better results with the 2-radiator setup mentioned earlier...3x120 and 2x120. You are planning on cooling a hot (older Core 2) quad and decently hot Crossfire config. You need more wattage dissipation than you realize.
If you use a single 3x140 rad, you should consider push/pull with some great fans...otherwise, 2 rads.