My ELAT advice to someone else on here was: have some sort of argument, back it up with decent critical analysis (complete with insightful comparisons between your chosen texts) arranged in a vaguely sensible way, stop ten minutes before time runs out and write a conclusion that draws the threads of said argument together (covering up the fact that you will probably have rambled off-track at some point) then quickly check spelling, punctuation and grammar, I suppose. I'd be inclined to stick to two passages because of the time limit, and it's probably easier to compare two things than three. Oh, and write clearly - better to be lucid than trying to pack in lots of long words etc.
I am of the opinion that you can't do very much to prepare for it though (or, rather, you shouldn't have to).