Check your Xbox 360's power supply while the console is turned on and displaying three red lights. Disconnect and reconnect the console's power supply if the power indicator light is green. If the power indicator light is not on or not green, the problem lies with your power supply. Contact Microsoft to replace the power adapter cord.
Shut off the Xbox 360 and let it cool down. The console may simply be overheating and needs to be moved to a better ventilated area.
Reconnect all cables on the Xbox 360 and turn it on again. If the console is still flashing three red lights, it needs to be sent for repairs. Red-ring consoles have a three-year warranty, so Microsoft should replace it if the Xbox 360 is still under warranty.
Open your web browser and navigate to the Xbox Online Service Center or call Microsoft Xbox Support at (800) 4MY-XBOX to submit console repair request.