As you may know, the omission of particles is extremely common in informal speech. Sentence 1 is the "correct" polite form, while Sentence 3 is a casual form where は is omitted. You would say 1 to your boss, and you would say 3 to your friend.
Sentence 2 is a mixture of both styles, which is also quite common in real-life situations. Politeness is not a black-and-white issue. When speaking respectfully to someone important, you should stick to Sentence 1. However, when conversing with colleagues, Sentence 2 is perfectly natural.
That said, I do think that Sentence 2 can be misleading and inappropriate as an example sentence in a textbook for beginners. Sentence 1 is always the correct choice as a polite form sentence to use in formal situations.