if you're hoping to write more books in the series, and you're hoping that readers will buy all of them, then you need the book number on the cover. Or else your readers are going to be looking at 2 or 3 or more of your books and won't have any idea which one is first without opening it, or trying to find a series order somewhere.
Your books may be purchased one at a time, but a lot of people don't like to start series until it's complete. Maybe they'll bulk purchase them, and then not have time to read for a week. Expect your books to have a life beyond someone looking at it on the purchase page on amazon.
Numerals are easier for the eye to catch, so I vote for number 3.