I bought a 2003 accord from a buy here pay here car dealer and the motor blew after 3 weeks. is it covered under tn. lemon law

Probably not. Tennessee has a Lemon Law but it only applies to new car purchases not used. More than likely your sales contract says the vehicle was "SOLD AS IS" in big letters. This means the car is sold in exactly the condition it's in with no warranties as to how it will perform. That puts the burden on the buyer to have the car inspected by a mechanic prior to purchase or take the risk that it might have problems. If it does not have those words in big letters on the contract then you might have rights under state or Federal law regarding implied warranties. If those words are missing it might be worth (depending on the cost of the car) to discuss your rights with a consumer or contracts attorney.