How to edit init.rc in Samsung Galaxy S3 i9300?
You have two choices here:


Edit the stock kernel (or the kernel you're using) to enable init.d support or edit the init.rc
Use a custom kernel with init.d support already enabled.


I would suggest the second option as is far more easier to achieve. Once flashed a kernel with init.d support, you can add all the scripts you want to the folder that will be executed at boot time. The folder is located in /etc/init.d and all scripts inside that folder will be executed in order.

For example, if you have:


/etc/init.d/10-startup-daemon
/etc/init.d/50-mount-extra-partition


They will be executed in the same order I've typed them. You can change the order by changing the number at the begining of the file.