Nick,
Personally I don't think looking at the MATS is necessary for S3, they dont expect you to know it, and it's a very dry document as well. S3 is more along the lines of applying logic and common sense to situations that the interviewer will present to you. Any decisions you make, make them based on reasons that you can explain, and be flexible if necessary in the wake of new information being introduced to the scenario. A little bit of ATC knowledge may help, but by the same token it may hinder. They are not necessarily looking for the answer a controller would give, but looking for the skills and decision making that are needed to be trained to be a controller.
To re-iterate what others have said, try and be as relaxed as possible, my S3 was the most relaxed interview of my life, it really helps you to think through your decisions. If you have made it this far, you're not far away
