If pain is there even after 3 weeks of filling, it means, it is not normal. May be the cavity is deep and tooth is already infected. Rather on the first instance, if you have pain on percussion, it indicates that tooth is already infected but diagnosis can be confirmed by taking xrays. Normally in such cases, Root Canal has to be done. In your case, you can get the xray done and if infection is there, whichi is the most likely cause, then you have to go for Root Canal treatment and above that crown has to be fixed. For more info you can read the following article from our website... http://www.identalhub.com/articlecategory-root-canal-therapy-17.aspx