Multiple buck convertors on one plug/power source
It depends on your power sequence. If 12V can come on before the 8V and 5V, and they don't interact, then this can work.
If however there is some control interaction between the 5V and 8V and the stuff powered by 12V (like speakers, motors, solenoids, etc.) then you need to make sure that stuff is turned off until the other rails are stable.
One technique is to make a power good signal from the lower supplies that can hold off (mute) the 12V-connected drivers until the rest of the power is stable and the system is ready.