Three red lights flashing means that there is a hardware fault.
Check your power supply light
Turn on your Xbox 360 console.
Take a look at the light on the power supply.
The power supply light is located on the end of the rectangular box in the middle of your power cord.
If your power supply light is green
Solution 1: Check your power supply cables
Unplug and reconnect your power supply. Here’s how:
Disconnect the power cord from the back of your console. Unplug the power cord from the wall outlet and from the power supply unit
Reconnect the power cord to the console. Then plug the power cord back into the wall outlet and the power supply unit.
Make sure that all three power supply connections are firmly secured.
Turn on your console.
Before you perform solution 2, I would suggest you to go through following links.
Following link is an interactive troubleshooting link.
Solution 2: Submit a console repair request
Your console needs to be repaired. There are no additional troubleshooting steps. To begin the online repair process, click one of the following links:
I am a U.S. customer
I am a customer outside the U.S.
This solution is absolutely FREE, so please rate and give positive feedback.