A few suggestions:
1. Let the clinicians guide you. They're used to people who are new to 3-whistle, so be honest with them about where you are and let them help.
2. Don't worry too much about missing a rotation. A missed foul is worse than a missed rotation.
3. In my experience, 3-whistle comes pretty easily with just a little practice, especially if you move a lot in 2-whistle. That means you have good ball awareness and are used to getting a good angle.
4. As C, don't be afraid to initiate rotations when the ball comes to your side above the FT line extended. Some folks (around here at least) seem to think that only lead can initiate a rotation.
5. At camp, when you're not on the floor watch others and try to anticipate the positive and negative criticism that the clinicians will give them. That tests your mechanics knowledge. But remember that theory and practice are quite different animals: it'll still feel a bit awkward when you start. And that's OK.
