How to properly format a table of contents?

I am making a table of contents, and it is formatted in a way where each chapter has a number, and then a sub section of a chapter is like 1.1 or 1.2 depending on how many there are. My question is, if I wanted to make it go one further, ie, a sub section of 1.2, what number would I use to display that?How to properly format a table of contents?
1

1.1, 1.2

______1.2.1, 1.2.2, 1.2.3