If you have Ubuntu with upstart, or one of the systemd operating systems, then your system completely ignores
/etc/inittab and it is a complete irrelevance.
Forget about runlevels.
They exist in systemd operating systems, but only as compatibility shims. The systemd documentation states that the concept is "obsolete". If you're starting with this on a systemd operating system, don't start there.
If you have a service manager, be it runit, systemd, perp, nosh, upstart, s6, or daemontools-encore, then it's already doing what you think you need
Take your stuff out of
On an upstart or systemd system it's really as much of a compatibility shim as runlevels are.
Put your stuff into an upstart job or a systemd service unit.
The latter would look something like
Description=Start the wibble daemon