It depends on your local landlord-tenant law

Usually if she gives you periodic rent payment, you should give her the notice one period in advance. In your case, it is one month since she pays you by month. However, your local landlord-tenant law may be different. I do think 3 month is too much though. As for the rent for May, if she does not pay, you have to follow the procedure as specified in your local landlord-tenant law in order to evict her. There is no such saying that you will have to deduct the security deposit as her rent.