There's good information at https://www.gov.uk/private-renting-tenancy-agreements/your-landlord-wants-to-end-your-tenancy.
There are several types of tenancy with different rules, but in all of them, the landlord has to give you a certain amount of notice to move out, and it has to give a specific date. "Three months from when a buyer is found" doesn't seem to fit that, so I would say you have not yet received formal notice to move out. This statement may have just been a courtesy to give you more advance warning. So for instance, if you are in a tenancy that requires two months notice, the landlord may have intended this to mean "I plan to send you formal notice about one month after a buyer is found." Of course, this is not binding; he could send you formal notice tomorrow if he changes his mind.
As far as I know, the sale of the house is really irrelevant in all of this. You don't automatically have to move out just because the house is sold; if you're not explicitly told to leave (with appropriate notice) then you don't have to, and you'll just start paying your rent to the new landlord. If you have a fixed-term tenancy, then the sale of the house doesn't shorten it.
I am not sure what the point is of getting angry at the estate agents. Your tenancy is a matter between you and your landlord, and the estate agents have nothing to do with it. They work for your landlord, not for you.