Each pixel of light coming from your computer is made up of a combination of Red, Blue and Green light of different intensities. As this pixel of light is so small your eye only "sees" the combination of those 3 pixels as the intended colour. (as opposed to just red, green and blue light) (This is also known as additive colour mixing)
The time complexity can only shift to 3 with the last instance. For the 2nd case, they need one parallel. However, 2 parallels are needed to sort with splitting into 3-way frames.
It decreases the number of passes even after breaking the collection in 3 by increasing contrast. So, the time complexity remains the same but the log is divided into 3 bits.