What is the validity of an oral agreement once a 3 day notice has been served.

I told my apartment manager that I would have rent in full on the 15th, she said that it would be alright for me to pay then but that the Property Management Co. requires her to serve a 3 Day Notice. This means that after the 3 days the manager/owner has legal right to change my locks even though I have made arrangements with them to pay on the 15th? What steps should I take to protect myself and should I be concerned about the validity of an oral agreement extending the time permitted to pay rent on a 3 Day Notice?
Thanks