I've got a couple of AC live DVDs and I watched them very carefully. Mostly he played bridge alone, a couple of times he switched to middle briefly for certain passages.
That doesn't mean other times he didn't use middle more, and especially in the studio, but on those shows I have it was 98% bridge alone.
I actually have a couple more DVDs now since that time, haven't watched them with that in mind. Maybe I will to see what he does on those. It's not like it's a chore to watch Albert!
"... I'm gonna show you the inside of me." Albert Collins