[TYPO3-english] Extend root template for custom pages

[Harish A]

my page tree structure looks like this:

ROOT
- PAGE 1
- PAGE 2
- PAGE 3

I defined a template in the root page. Which is actually a layout, contains
header footer and a subpart for main content(´MAINCONTENTSECTION´).

Now for each page I have different html template. I want to extend the root
template for each page so that the custom html template of each page comes
inside the ´MAINCONTENTSECTION´ subpart. How to accomplish this with
typoscript?