Without reading your lease or the letter the landlord sent you, the following is some general information:
From your question, it is not apparent if you paid rent for June 2012. If you have then you are not delinquent, although you may have some late fees due. However, it appears that you not made those payments. Therefore the landlord is justified in having you evicted. To do that, however, the landlord needs to go to the J. P. court and file an eviction lawsuit. In that suit the only issue is have you paid the rent.
Texas property code §24.005 states "the landlord must give a tenant who defaults or holds over beyond the end of the rental term or renewal period at least 3 days written notice to vacate the premises before the landlord files a forcible detainer suit, unless the parties have contracted for a shorter or longer notice period in a written lease or agreement."
Therefor to answer your questions your landlord must provide you at least 3 days notice, and comply with §91.001.
If you Have paid your rent, I suggest you contact a lawyer immediately. It appears that the landlord is hoping that you will vacate, immediately. Good luck with your claim,
Has the landlord accepted the June and July 2012 rent payment? If he has not, then
As a practical matter, the landlord doesn't really expect you to be gone in 3 days. They will file a suit in forcible detainer and then must serve you. Then you must go to court as scheduled which could be a week or more later. At that time ask the judge when the writ of possession will issue. You will have at least that long to move.
This comment is given for educational purposes only. No attorney-client relationship exists between us.