I agree that diodes would be the simplest way to do this...about a half volt drop for each diode in series.
If you need better regulation, another option would be a DC/DC buck converter (Similar to this:
https://www.mpja.com/LM2596-Step-Down-Adjustable-15-37V-DC_DC-Converter/productinfo/30148+PS/ ). These Chinese devices are available on eBay, Amazon, and other places for a couple of bucks. You will need at least a couple of volts difference between the input and desired output voltage. I would also be careful to filter the DC out of the converter to ensure it is clean and doesn’t modulate your transmitter carrier with noise.
You could also use a 7810 regulator chip, though if your input is 13.8 volts you might not have enough of a voltage difference for it to be reliable.
A fourth option might be possible. Many power supplies have an internal adjustment to set the output voltage. If your PR3 has this, you may be able to turn the output down far enough to meet your needs.