Part III will entail supporting your design choice....So make sure you thoroughly study your submitted design for part 2 before embarking on part 3. Things like scalability, session management, choice of BMP OR CMP, choice of persistence, mechanism of persistence, load balancing and security in ur architecture are potential questions asked.
>>>So far I'm thinking, I don't need any preparation all I need is back up what I did and why I did it.
Part III is fun. You don't need any preparation but you do need to have your approach in memory. The idea behind Part 3 is to put words on 'paper' that will be read as the examiner looks at your UML diagrams.
One thing I didn't do is read all the questions before answering them... that might help you structure your answers and stop repeating yourself....
In Part II you get graded on the diagrams. Part III is the words... to back up the diagrams.
Good luck! and take it soon whilst Part 2 is still inside your head.