You need to format a partition as ext3 before you start using it. So it would have been done in chapter 3.
However, if you currently have an ext2 partition you can convert it to ext3 at any point you want.
Have a look at the following page from the Beyond LFS book, it'll explain how to do it:
http://beyond.linuxfromscratch.org/v...tlfs/ext3.html