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It is strange that all three units have the same problem. I am assuming that when you tested your three units that you did so by attaching them to your camera. If all three are bad, it could be that the problem is in your camera. Let's disregard the ready light for the moment because the ready light could be bad or your flash is only partially charging. As an experiment, turn on one of your SB-600's and after allowing time for it to charge, short out the contact pins on the flash shoe. If it flashes, it could be the camera at fault.
The SB-600 is an older unit. If you have three of them, I will assume that you are using them professionally and they have had heavy consistent use. As a result, your speedlight's capacitors may have reached the end of their life span.
Let's look at what may be happening. By charging, it means that the batteries in your speedlight are supplying a low voltage to a capacitor bank that charge to a higher voltage that is sufficient to fire the flash tube. All capacitors deteriorate over time and use and will lose their ability to charge and maintain a charge. Given the age of your equipment, that may be the case. If you have the knowledge and equipment, I would replace the capacitors on one of your units and see if it works. If not, I would send one of the units into Nikon for a free repair estimate. They can be reached at 1-800-645-6687 9AM-8PM EST, Monday to Friday.
put in a fresh set of batteries. The ready light won't turn on until the flash capacitor is fully charged. And if your battery is weak, it can take very long (minutes!) to fill the capacitor. You still can fire the flash even it is not ready, it will just dump whatever power it stored in the capacitor.
If one cell has died, there will be enough power to power up the flash, but not enough to charge it reasonably fast. It may also stop the other cells charging up properly if they are charged together. I've had this problem with poor quality rechargeables before. I think flashguns put pretty heavy demands on batteries.
Trigger coil or a flash tube. Flash tube is easier to replace so try to replace that first. Caution before you do anything else when you open the unit locate the main 330v capacitor and discharge it before you work on the unit !