I wouldn't worry about it too much - 1/2 an amp is more than enough to drive 3 muffin fans at full speed. You'll have to wire them in parallel so they all get the full voltage of the power supply.
For $24 in parts, though, you can have independent speed control of all 3 fans with the motor controller at Bakatronics - It's a PWM circuit. Basically, instead of increasing the resistance to the current flow (Dissipating power in the form of heat), it switches the fan on and off really fast. The longer the "On" cycle vs the "Off" cycle, the faster the fan turns. No heat Not that a muffin fan is going to roast a 3 watt pot - CPU fans draw less than 1/4 amp - or 3 watts at 12 volts.
You can run the speed controls with your current power supply, or just use a wall wart.