You are going to need to rework some of the content in your current topics that contain nested content. You have two choices:
- Move each topic into a separate file and aggregate then for delivery using a DITA map
- Use nested topics within your current files to create the hierarchy that you want
These are the two DITA architectural mechanisms for creating a navigational hierarchy. Either of these approaches will yield the following result; do note that I have added numbering for the chapter:
Chapter title A
introduction text
title A.1
text....
title A.1.1
text....
title A. 1.1.1
text....
title A.2
title A.2.1
text....
title A.2.2
text....