Well I have one main domain domain1.com this is the main domain. I also have 2 others that I appoint to the hosting service. I then have domain1.com | domain2.com | & domain3.com all the same site. Now when I click a link in domain1.com it will be like this domain1.com/link.html but when I click the link on domain2.com it is domain2.com/link.html. Now all 3 domains are indexing the same page just with different "domain.com". Is this considered spam, I know what I said is confusing but someone will understand :-P
If the same content is on more than 1 domain its duplicate content. A few pages are OK (only 1 will show up in the serps) but more and you face a ban.
That doesn't have to be so.. Google will look at the ip adress of both sites.. if those are the same (domain just pointing to the same hosting) I don't think it is considered duplicate content. If you have seperate hosting for seperate domains and you serve the same containt, that would definately considered to be spam. (I have experience with the first set up, and we don't suffer a ban...)
even in same domain different directory, different sub domain. it can be concerned as duplicated content.
I'm not saying the sites will be banned. In most cases when a search engine finds dup content it will just choose one trusted source and filter the others from certain search queries. In this example a search will probably bring up domain1 and not domain2 or domain3. If you have loads of dup content then you face a ban.
You will have duplicate content issues if you submit all 3 sites to search engines. If you only submit 1 domain to search engines, you will be fine as long as the other domains do not get found. As soon as the engines find the other domains though, you will have to create separate sites for your domains.