"3. Do not create 301-redirects, but rather choose one of the following 2 methods: A. At the Domain cPanel, FORWARD the Domain that is deemed "lesser" to your Main Domain, for in this way there is not negative SEO affect upon any domain at all, since all metrics are attribute to the Destination Domain, not to a Domain that has no site or other characteristic associated with it; B. Create a single-page "referral" website for each secondary Domain, inviting visitors to proceed to your Main site. Also on this single page portal, you can have some AdSense placed to captialize on the potential divergence of traffic to earn some revenue. This minimal content and a single "outbound link" to your Main website is not enough to create a violation, and by the addition of AdSense, you will contribute to a more dynamic Content aspect that will diminish the fact you are promoting an outbound link yourself.
* If your Domains are generic, normally occurring terms or relevant words-based, forwarding the to a Main Domain if not in use is a smart way to "activate" them as far as Search goes (as they will come up if the User types in similar words or phrases) and can then provide an additional degree of referring traffic organically to your site."