My son's xbox is showing 3 red lights

e74 or rrod error means that there is an hardware issue.
What you need to do is a very simple and cheap repair.
look on ebay for x clamp repair kits, these are just a few bucks and most sellers include a good guide in how to do the mod/fix.
what causes the rrod or e74 error is the X shape clamp on the underside or the mother board bending and mis-shaping causing the ball soldering on the gpu/cpu slightly melting and forming hairline cracks in the solder.
The x-clamp kit will apply more pressure to the gpu/cpu and a little trick of disconnecting the fans and turning on the xbox and allowing it to overheat will re melt the solder ball joints to fill the hairline cracks.
Depending on which chip is the issue you may also need a 2000 watt heat gun instead of using the over heating method (the gpu chip).
Replacing the heat pads on the heat sink with thermal paste which comes with the kit will keep the gpu/cpu cooler and you wont have this issue again.
Please make sure you buy the kit which comes with heat treated nylon washers as the steel spring washes fail!.
The repair takes around 1-2 hours
I hope this helps in fixing you xbox.