Generally, good Div III’s grow out of Division II. That is, they build on what you have done in Division II where you are developing skills and abilities and articulating questions that lead to your Division III project.
One way to prepare is to reflect on what you have accomplished in Div II and what questions remain for you. It ought to be something you are intensely interested in, as you will be spending the better part of 9 months working on it! As you put the final touches on your portfolio and reflect on the meaning of what you have done through your retrospective, see where that leads you. You should end your retrospective with thoughts about Div III and end your Div II final meeting with a conversation with your committee about how to launch yourself into Div III.
Because finishing Div II helps you start Div III, it is best to have your final Div II meeting at the end of your last Div II semester (May or December) rather than the start of your first Div III semester. That way, you can use the summer or January to begin your reading, piloting, site visits… whatever will help you be ready to start Div III the next semester!
Also – it is never too early to visit the Div3 at Hamp blog and pages!!!