Why, in the same light bulb, three of the six leds flick, and the other do not?
The reason this happens is that in some of the cheapest and crappiest LED lights large batches of the LEDs are all wired in parallel with each other. Each of the LEDs that are in parallel has a slightly different voltage threshold at which it lights up. Due to the parallel connection of all the LEDs the circuit causes the same voltage to be applied to all the LEDs. The LEDs with the lowest thresholds will be the ones that light the brightest whilst the ones with the highest thresholds will be the dimmest or will not light at all.

Flickering will happen with some LEDs that are on the very edge of being turned on. This flicker game is the fact that as those LEDs right on the edge that are turning on will have slightly increased temperature which changes the LED's threshold voltage some. This change in turn causes the current to change a small amount which pushes the LED on other side of the on/off edge. This results in a temperature decrease in the LED and starts the cycle over again. The relatively slow nature of thermal changes is what makes the flickering at a visually detectable frequency.