It doesn't need to be 7.5V, as long as it can provide at least the minimum 3A that you need, the PSU itself can be 12V. Just regulate the voltage down to what you need it to be. I drive LEDs with 12V and 24V supplies all the time. Add the appropriate resistor for the voltage and current needed and you're good to go. Granted, this means you can't use a stock Arduino by simply plugging it in to the PSU. You 'll have to either build a separate circuit to regulator that voltage, or cook your own Arduino-clone. Neither are particularly hard to do.