This is the rating for the continuous current.
If you use 6x multiplex, it's only 16.6% modulation.
But much simpler is to increase the resistor so that the 75mA is spread over the 8 LEDs, so that is just about 9mA per LED.
At 3 volts the currents are lower than 20mA, so it can work 50% or 100%.
This is just nonsense, you can't run these chips at 3V from a 5V arduino without level shifting. Please remember this poster is a beginner, describing half baked answers is not helpful. The brightness of an LED depends on current not voltage.
there are 10 to 20 factors that limit current, otherwise the IC (Arduino) would really use 3 Amps
That is simply not true.
The lowest gap, 0.5 volts, has really large increase in current. What becomes the limit then are the MOS channels, to the current distributes. They work like resistors and that's OK for them. 120 ohms, but i do not know it can change due to voltage. not a ohm resistor.
This is so poorly expressed that it makes no sense at all.