Using a 12ohm resistor is not the best way of reducing voltage for a motor. You would only get 1A if the motor voltage were very low. A 3V motor would only get 0.75A and a 6V motor would only get 0.5A.
A much better way would be to use a ‘buck’ converter. These devices used the ‘switched mode’ technique to reduce the voltage with very little (about 5%) loss of power. These types are typical of the devices you can get. All are from Hong Kong, are inexpensive, and I can vouch for their quality.
A simple buck converter giving up to 3A is this one.
http://www.ebay.co.uk/itm/1-DC-DC-Buck- ... 5d44cfdcc7If you exceed the 3A max, the unit will shut down for safety.
This is the same type with an inbuilt voltmeter
http://www.ebay.co.uk/itm/DC-4-5-28V-To ... 565fd043f7Note that with both of those you will lose about 2V from the supply, so you could only get 10V from your PS3 supply.
A better type to use would be this one, that also gives a variable maximum current adjustable from 100mA to up to 5A. It is a ‘buck/boost’ type so you can either increase or decrease the supply voltage from 0.8V to 28V
http://www.ebay.co.uk/itm/5-30V-to-0-8- ... 3a8900d308Finally, the Bolls-Royce of these devices, a buck/boost converter that from your supply would give 0.8V to 29V with an adjustable current up to 3A (5A with a heat sink), with built-in voltmeter and ammeter. My son uses one of these as a battery charger for his fishing bait boat – it’s the only way to charge a 12V battery from another 12V battery.
http://www.ebay.co.uk/itm/DC-Boost-Buck ... 2589b65615If you wanted easier adjustment, you could replace the preset controls of any of these items with standard pots and control knobs.
For more alternatives, go to Ebay and search for 'buck converters' or 'buck boost converters'.
I hope that helps.
CORRECTION!
Since replying, I have looked again at the specs for the last two converters. Although advertised as buck/boost converters, they state in their specs that they can only reduce the voltage. They are therefore NOT boost converters. From a 12V supply they can only output 10V. If you want to go higher than 10V, the best thing would be to use a boost converter first to raise the voltage. For example, this converter could lift the voltage to 30V
http://www.ebay.co.uk/itm/XL6009-DC12V- ... 35d119584dYou could then apply that voltage to the buck converters to get any output from about 2V to 28V.