To modify the meters for 3.3 Volts, I unscrewed the case and found a fixed resistor and a POT in series with the input terminal.

Since the meters were build for 10 Volts full-scale deflection, I used a fixed resistor value approximately 1/3 of the original one.

Then I connected a DC voltage source set to 3.3 Volts and adjusted the POT until I had full-scale deflection.

You can get all three boards as a kit here: