Yes google will take it as a Duplicate content & it can be harmful for your websites. So for SEO purpose I would like to suggest you to Optimize your main Site only instead of all sites. Must use cronical tags but carefully..
"cronical"? probably meant to write "canonical". Nevertheless, as the question appears to me, you only have content on one site with two other domains forwarding to the main site. Should this be the case then rest assured it is not penalizable because it is almost a standard nowadays to do such things.
There is lot of confusion with IP address for SEO.According to search i can only say that a dedicated IP will not boost your site ranking.
To the question the value of dedicated IPs webmasters had at that time,
According to some Webmasters ...
Craig said, “Google handles virtually hosted domains and their links just the same as domains on unique IP addresses. If your ISP does virtual hosting correctly, you’ll never see a difference between the two cases.” In 2006, Matt Cutts, who still leads the webspam team at Google, mentions that article in his short blog post debunking the dedicated IP myth, where he says,
“There is no PageRank difference whatsoever between these two cases (virtual hosting vs. a dedicated IP).”
If you have mulitple domain hosted on same IP then you should have unique contents on these domains.
This effects the SEO.
And I believe that hosting multiple domains on one hosting account is not bad at all indeed, rather cost effective. Corrections are warmly welcomed :)