Try it.
Assume the LED voltages at 20 mA are approximated by 3.5, 3.5, 4 and 2.5v (the unspecified low end of LED voltages). With three LEDs this is 4x3 values.
Then add voltage distribution, assume 12.8, 13.5, 14.4, 14.4, 15.
Then add resistor tolerance, assume a normal distribution with...