What will happen?

[AC Question]

What will happen if the phone is off then I press and hold Volume Down + Power button + Home button?

FOR GALAXY SIII ...

<3

 

[Tim1954]

The device will boot into Odin mode. There you can flash firmware and generally tinker with software via a pc.
It's best to avoid this unless you know what you are doing.
If you're looking for recovery mode, you use the volume UP instead. There you can wipe the cache partition, which is a good thing to do from time to time...