If you press and hold the power button for 3 seconds after you unplug it from the AC cord it will not discharge the power supply?

Pressing and holding the power button for 3 seconds after unplugging a device from the AC cord typically doesn't discharge the power supply because the device is no longer receiving power. The power button may only initiate a shutdown or reset sequence if the device still has residual power in its capacitors or battery. To fully discharge the power supply, it may be necessary to remove the battery (if applicable) or wait for any residual charge to dissipate naturally.