Telling the child different amounts of information in different ways all through their lives is the best option. Just as you will not send a 3 year old child to college, you will not tell a 3 year old child something only a college student can understand. So you have to tell a 3 year old what a 3 year old can understand and keep adding to that as s/he grows older.
Document as much of the adoption process as you can- make notes, take snaps, write down details on your calendar/diary/phone as they happen. You cannot possibly remember everything that happened, so the more you document, the more information and tools you have to share with your child.
Do feel free to send in your opinion / suggestions / stories related to ‘Disclosing the fact that a child is adopted‘ to us at firstname.lastname@example.org and we will surely add it to this section.