Start with the Bunshou no Katachi chapter. These are 1-2 sentence exercises. Master the grammar connectors first. (e.g., Distinguish between "sore ni taishite" vs. "sore ni yotte").
The book teaches three specific skills. Practice them blindly:
You can buy the official eBook for Kindle (or Kindle app on iPad/Android). Search for "Shin Kanzen Master Reading N3" on Amazon Japan or your local Amazon store.
Should you hunt for a free PDF? Only if you absolutely cannot afford the book or live in a country where shipping is impossible. The physical copy's layout (which forces you to flip back and forth to check answers) is actually better for memory retention.
Final recommendation: Buy the paperback from Amazon. But if you need a PDF for digital flashcards or printing specific pages, check your local library's digital loan system or buy the official e-book.
N3 introduces more abstract writing. This chapter helps you navigate: