Instructions
Turn on your Xbox 360. Wait to see if you get the three red rings again.
Take notice of the light on your power supply. The power supply is the rectangular box in the center of the power cord.
Turn off your Xbox 360 and unplug the power cord from the wall. Also disconnect the power cord from the Xbox 360 and the power supply. Reconnect everything and turn the console back on. If you still have the three red rings and the light on your power supply is green, submit a console repair request.
Plug your Xbox 360 into a different outlet if the power supply light is red. If you plug into a different outlet and the light turns green, something is wrong with the previous outlet. If the light is still red, make sure the power supply has proper ventilation wherever you have it placed. If after doing that you still have the three rings and the power supply light is red, contact Xbox support for a replacement power supply.