they sent a little leaflet in the box register product US but i live in the UK i rang canon uk up 2 day and they said its ok to register but at moment they have website update
If I were in your shoes, I'd get that in writing.
also the magic lantern requires to have the latest firmware.
I'm aware of that - I use ML on my camera. The 5D3 is the same camera whether it's sold in the UK or the US so I can't see why the firmware would be different - "colour" is spelt incorrectly on my 550D's menu.
Did you see that ML for the 5D3 is only in its infancy so you'll have very limited benefits using it?