Recently a 3 day notice was posted to my door. The property owner was also supposed to mail one but instead she just put it in my door inside of an envelope. Is that legal to do instead of mailing it?

Yes it is legal to just do that.
Was it for non payment of rent and what state?

Maybe, maybe not. Depends on your local laws. But the question is really moot since you are aware of the notice. What's going to happen is the landlord is going to file for eviction after the three days. A hearing will be scheduled and you will get a summons for that. You will have to pay before the hearing or face eviction.