2. 4 coats are better than 3. Time and cost are the the reasons. The more the merrier, just let the coats cure before you recoat and lightly sand between coats and wipe with tacky cloth, that will give you a mirror finish.(Don't sand the last coat, you wouldn't believe how many people do).
3. Take out everything that was there previously, you want to start with a fresh foundation, 3/4" subfloor is great place to start.
4. If you have an uninsulated crawlspacew below you, a concrete slab or an unfinished basement put down a vapor barrier before the hardwood flooring. This will protect your hardwood flooring from condensation that can twist and "cup" your wood floor.