I am wondering if our site is being penalized by Google for having multiple domains pointing to the same site.

We have one site that features product pages in sub directories. The main domain points to the home page of the site and then we have two separate domains for each of our products pointing to their respective sub directories. When these product domains are entered into the address bar of a browser, they redirect to the sub directories of the main website. Is this something that will hurt us with Google or other Search Engines? I know that only the primary (target) domain has a real presence in the search engine indexes but I was wondering if there was any negative aspects to having multiple domain names pointed at one site.

Thanks!