Can a landlord evict if they did not serve you a 3 day notice

My mother rent a unit under a rent stabilized building. The landlord, a few months ago, offered my mother $10,000 to leave the premises. My mother did not follow up with that because she is happy where she is and did not want to move. My mother has a lease that started on the 18th of the month with the previous managers and she has consistently for the last six years paid her rent on or about the 5th of each month. She is also a tenant who receive Section 8 housing the landlord excepted the Section 8 portion but when my mother attempted to pay her portion she was told that her money was not going to be accepted because she did not pay by the 1st of each month as she was supposed to. My mother has always paid about the 5th of each month because my younger sister is disabled and receive Social Security benefits which is not available until the 3rd. Her landlord stated that they served her a three day notice on the 2nd. My mother received no notice and was confused when they would not accept her payment. The landlord has filed eviction process for non-payment of rent what are her rights and how can she prevent losing her home and Section 8 due to the landlord attempting to remove her from the property illegally. The landlord is removing all of the original tenants and then raising the rent a substantial amount. The building is rent stabilized and removing the tenants allows the landlord to receive the rent that he chooses once a new tenant moves in. What can my mom do if she was not served a three day notice? The landlord has consistently accepted the payments under these terms via verbal agreement but now is using false information to evict her from the premises.