Except for one person, no one asked you to send tax returns WITH N400. The person who told is for marriage based GC? Is your 3 year marriage based GC or 5 year PR based GC? If it is marriage based GC, sorry I do not know the requirements. What I mentioned and know for sure if for 5 year based GC. Please use the instructions on the form as a final guiding information for you. I always go to forums, and Internet for information, but use the USCIS website as a final piece of confirmation.
Why do you have two threads for same topic ?