Actually, TS3 was a 7.1 mix in the theaters - one of the first of its kind. Only select theaters had upgraded in time to produce all 7 channels, but that's a separate story. The 7.1 mix on the Blu ray is supposed to be bit for bit identical to the theater soundtrack.
The disc also includes a separate 5.1 mix, which probably sums the two rear channels and places them in the side surround channels, as ClubNeon stated. It is a totally separate soundtrack from the 7.1 track, according to the disc specs.
Since it the center channel, and not a surround channel, I wonder if there is an error in the 7.1 center channel mix or in the decode process peculiar to that mix that is causing the scratching. But I bet it's not a down-mixing issue.
I bet its just a really hot mix that sent your center channel amp into clipping.
