you're screwed, call microshaft for a refurb, or if you have warranty for the store i would go with the store cause microshafts refurbs might do the same 3 red lights.
To receive instructions for obtaining repair or replacement warranty services you must call:Within the U.S., U.S. Territories and Canada: 1-800-4MY-XBOX (1-800-469-9269); TTY users: 1-866-740-XBOX (1-866-740-9269).
Edit: shipping might take a while too, so shipping might take all of halo 3 beta time
if you want a short term remedy, wrap it up in 2 big towels all around, leave it on for 30 minutes, it will get really hot, take off the towels and let it cool off, turn it on then it wont give you 3 red lights anymore, but this is only a short term fix, tomorrow it might give you the lights again.
http://youtube.com/watch?v=b0LHbNPo1qY