I would choose the 7.5V power supply. Then use two LM317 regulators as previously suggested. Use one regulator to drop the 7.5V to 3V and the other one to produce the voltage for the LEDs. The LM317 is easy to use. The datasheet will give a simple equation for setting the output voltage based on two resistor values.
The drawback of linear regulators are that they dissipate heat. Expect to radiate about 1W of heat from the 3V section. Use a heatsink and/or a fan. Since it is a prototype, you probably do not have to run for long intervals.
There are newer and better regulators than the LM317. However, this device is easy to find. The reason for the 7.5V supply over the 5V supplies is that the LM317 requires several volts of headroom. The 5V supply would not have enough headroom to generate the 3V output.