To make the secondary domain, the primary domain, unpark it or "un" addon it first in the primary domains control panel. Then go to WHM > modify an account and change the primary domain name to the secondary domain name.

You now have the secondary domain as a primary domain. Then you can park the secondary and tertiary domains on the primary domain.

Then from there if you want to setup a 301 permanent redirect like you are trying to attempt you can unpark these domains and make them addon domains then use .htaccess to make the 301 redirects.

As far as the correct syntatx to use in .htaccess I am not able to help you there you'd need to make sure you are using the correct rules for the version of Apache you run as I don't think rules for 1.x work on 2.x and vice versa.