Creating a .dd image of a Galaxy S3 without rooting the phone
It's impossible to obtain block level access to a block device without root, since all of those files grant the write permission only to root. At least I have never seen it any other way and I doubt that the S3 is an exception here.

It's not because it's technically impossible, the default permissions simply forbid it. A this is done for a pretty good reason: It would be a major security issue if this was possible: The normal user could get a dump of all the filesystem data.