Cyclone's idea is a good one. My only objection is too much air inflow. You don't need that much fan unless you running the drives to full capacity all the time. It real depends on what you are cutting. If the drives where loaded to full current capacity they would disapate a fair amount of heat. I don't know what FETS they use in the bridge section of the drives, but modern FETS would have an average of .06 or so RDS. So at 20 amps they would disapate around 50 watts. But I doubt you will be running the drives at that load all the time. A healthy chunk of aluminum with one fan for three drives will work fine. I have three drives mounted on a old heat sink out of a home stereo receiver and is works great. I tend to lean away from all the fans unless you need them. If your cutting wood, you have to deal with your control box filling up with dust in a hurry.
Just my 2 cents.