how to use Audit trail concept in 3 tier architecture?

Okay, you need to add a handler for when you make changes to your data, so that you can track the details. You need to inject (add something external into internalized code) a handler for those changes, either by using an event-driven approach or by literally placing it between your I/O (your http hander for web applications) and you data persistence logic (ORM, SQLConnection, file serializer, whatever). This additional object can handle the logging of the details of the transaction.

I'm sorry, I can't provide any code. I don't know your implementation strategy, your data access strategy, or a blessed thing about your application.