Yes, assuming you mean you're hosting the same content on 3 distinct domain name based websites (Example.com Example.net Example.org) and not 301 redirecting 2 domains to the 3rd or actual site.

Let me show an example.

example.com, seoexample.com, designexample.com are all pointed to to the same content. Let us say all three are pointed to public_html which means all use the same content. And all are indexed by google or others. How does that affect to duplication?

Unless two of those sites are 301 redirected to the third one, if they all share the same content then yes, they are all working together to hurt each other.

Thanks,

I am now not sure which one to use. I have three domains. one domain is 7 month old and has not PR and does not contain any keyword with what I am going to cover. Second domain is 3 month old, has PR 2 and contains two keywords. Third domain is a root domain of my account and has registered new but contains three keywords of what I am going to cover but nor PR or indexing.

I need suggestion which one to use a base and redirect the two to that one. I want for a long term so when I build links, I do not lose time switching between three.

I know best seems the second one but not decided yet. Any help is appreciated.