What are the difference among terrible,horrible and awful? All are the same meaning: very bad or unpleasant.

They all mean the same thing. There isn't really any distinguishing between them. Example: "Don't go to that restaurant, it's awful. The food there is horrible and the service is terrible." "Don't go to that restaurant, it's horrible. The food there is terrible and the service is awful." "Don't go to that restaurant, it's terrible. The food there is awful and the service is horrible." "His sister died? That's terrible/horrible/awful!" All the words are interchangeable, so don't worry about it.