Strangely, I found Part 3 the easiest - I think I only made one mistake on it, compared to about 6 mistakes on Part 1. Parts 1 and 2 are your first exposure to a new challenge, whereas Part 3 is more a test of multitasking and prioritising ability, and you can almost leave the individual tasks down to your autopilot to manage, because you know how to deal with them already.
Best advice I can offer - if you make a mistake, you haven't failed. If you make many mistakes, you probably will. So when you make a mistake, forget about it immediately and refocus. Good luck everyone!