It might seem daunting at first, especially if you have never been inside a games console or other electronic device but its really quite simple to fix your Xbox 360. Like any project the task can be broken into easily manageable steps.
1. Find out what the error code is.
2. Get yourself a good, comprehensive, well written guide.
3. Get the parts and tool necessary
4. Tough for some, including me, follow the instructions!!
Do all of these things and you will soon have your poorly Xbox 360 back on its feet.
Ok, you've got 3 red lights flashing and you've asked the question 'how to fix my xbox 360' - that's good, at least you've not given up before you get started. The first step then is to figure out what is wrong with your Xbox 360. To do this, look at the power LED - the ring of death as it is now - and the flashing sequence will tell you what is causing the problem. The red ring of death is caused by overheating, this is a known problem with the Xbox 360, but its difficult to tell what component has actually failed. However, the different patterns indicate numbered error codes which will help define the problem further. A good guide will list the error codes, as well as tell you how to open the console without damaging it, and how to fix the problem you are having.
That leads me on to the guide. There are many guides available online for immediate download as well as a lot of bad advice and downright dangerous stunts that can permanently damage your Xbox 360 or worse. However, I have found a great website that gives a comprehensive comparison of the best guides, as well as tips, hints and even sample videos, so that you can have the best chance of success. Its listed in my signature at the foot of the page.
Ok, once you have your guide, but you are still pondering 'how to fix my Xbox 360' - well next you need some tools. Don't worry, you probably have most of them already, all you really need are some screwdrivers, a pair of pliers, possibly a small drill, some nylon washers and some assorted screws, and finally some thermal heat paste. Again, a good guide will list the exact items but the total cost shouldn't exceed about $10
Time to get down to it then. Armed with your guide the whole job shouldn't take more than 1-2 hours and its almost foolproof. The basic steps are
Open the case
Remove the DVD drive, fan and mainboard
Remove the X-Clamps - the guide will show you what these are
Clean up the CPU/GPU and apply new thermal paste
Put it all back together
Do a 'secret trick' to make the whole thing work
Test
Enjoy your fixed Xbox 360
So there you are, a simple, cheap fix and best of all its permanent. You don't need to send your Xbox 360 away for weeks or have expensive repairs done at a store, an hour or two and a good guide will get you back working again.
When my Xbox 360 suffered from the 3 red lights error, I did the legwork and found the best guides around - remember they are not all good or even suitable - and put together this website to help others have the same success I did. Good luck.