You probably can’t really drive 3 LEDs in series directly from each output.
The absolute maximum voltage for the microcontroller is 5.5 V; do not exceed that value. The actual voltage output may be up to 0.8 V below the input voltage, so you may get as low as 4.2 V output– only 1.4 V/LED. If this is the case, the LEDs may not light up at all, even without a resistor. On the other hand, the voltage drop on the output may be lower, so it’s possible that you’d get up to 1.83 V/LED, which may be enough to overdrive them. So, you are trying to match the 3X the forward voltage of the LEDs– a not very specific spec –to be less than or equal to the power supply, less the voltage drop of the microcontroller– a not very specific spec.
You’re welcome to try, of course, but I wouldn’t guarantee that it would work.
hey, if you’re up for a little coding adventure, you could charlieplex 6 of the i/o lines on the scanner to drive up to 30 leds. i did it once on a minipov with 64 leds, (sinking 8 leds on each of eight pins), and didn’t blow out any of the i/o.