This file is indexed.

/etc/rc3.d/README is in sysv-rc 2.88dsf-59.9.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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The scripts in this directory are executed each time the system enters
this runlevel.

The scripts are all symbolic links whose targets are located in
/etc/init.d/ .

To disable a service in this runlevel, rename its script in this
directory so that the new name begins with a 'K' and a two-digit
number, and run 'update-rc.d script defaults' to reorder the scripts
according to dependencies.  A warning about the current runlevels
being enabled not matching the LSB header in the init.d script will be
printed.  To re-enable the service, rename the script back to its
original name beginning with 'S' and run update-rc.d again.

For a more information see /etc/init.d/README.