The three oscillators simply turn on a colour then turn off the colour at different frequencies. They do not gradually brighten and gradually fade, they just turn on and off.
Sometimes all three LEDs will be off. Sometimes only one colour will light. Sometimes two colours will light and add to make a 3rd colour. Sometimes three colours will light and add to make a 4th colour.
Since the oscillators are at different frequencies then the pattern will appear to be random without repeats. But if you look at a single LED you will see it turning on and off.
I made some Mood Lights that use triangle-wave oscillators to slowly brighten and slowly fade three colours at different frequencies. They add to make millions of slowly changing colours and brightnesses.