Owning multiple URLs can dilute link popularity. For example, rather than 50 links to your intended display URL, the 50 links may be divided three ways among the three distinct URLs.
Link popularity is important to websites, because the more traffic you get for your search term, the more reputable your website becomes. So with three sites basically competing for the same terms, you are actually hurting your chances at gaining in Pagerank with any of them.
I know, you wanted more sites so that you'd have a better chance at getting more traffic. As long as you use different search terms and keywords for the same product or service you should safetly be able to have three or more sites (so long as they don't each have duplicate content).
The thing is, if all those search terms are on just one site, then you will pool all that traffic, thus gaining reputation with the search engines as the site to beat for all of your search terms.
Something to think about!