The ET3 is the Reply to your ET1.
If done properly the ET3 will tell you what the other side has to say about your case. It is important that you get a good understanding of what they are saying to the Tribunal.
You should identify where you agree with them and where you do not. Where you do not agree is where you need to focus your efforts. This is where the facts will meet the evidence and the law. Can you convince the Tribunal that yours is the better case?