Jack Allison wrote:
How does the 3-bladed prop get rid of the yellow arc?

The yellow arc in the middle of the green arcs is an area of
"resonance". That is, it can cause vibration in the airframe. With the
two blade prop operating in that rpm range, a frequency exists where the
whole airplane is in synch. Ever see the video of the Tacoma Narrows
bridge oscillating in the wind? The structure becomes "fluid-like" and
distorts in proportion to the amplitude/strength of the wave.
The three bladed prop has a resonant frequency outside that of the
airframe/engine/prop system.
This is the best answer I can give. I understand it, I may not be
explaining well.
I believe Jim Weir could probably give you a better expaination. He's a
teacher and very good at reducing technical things to simple english.