Wow, I missed this. I did not know such a promotion.
As they mentioned, you can host unlimited domains, unlimited FTP users, unlimited MySQL databases, unlimited Email addresses in DH. If your domain is registered elsewhere, you can host them in DH by modifying DNS.
Unfortunately, there is no way to check whether the promo code is active or not. The only way is to check your billing price after you key in the promo code in the payment page. I guess you can check with DH support via contact form for Lunarpages and Registerfly case.
$50 off and 3 free domains with code: [color=#CC0000]DH3[/color] [url=http://dreamhost.com/r.cgi?296804/signup|DH3]Sign Up NOW[/url] or More Codes Here