Ok, another try (before I give up :))
In this exact situation (for now, I don't need to know in other circumstances..), let's say my multimeter show: 20 mA
3.3V, 20 mA, no resistor between Arduino's 3.3v pin and LED-A pin. There may be a resistor between LED-A pin and LEDs, or not, I don't care.
So for 5V, using a PWM output instead of the trimmer, I would just need to add a resistor of value (5-3.3) / 0.02A = 85 Ohm, between the PWM output and LED-A pin.
Am I wrong once again?