What you want is the "Legacy Package CD" or "Legacy Package Download" -- the CD comes with a nice jewel case and Starfire artwork or you can download the package version. As mentioned on the order page, the Legacy Package contains all of the 1st, 2nd, and 3rd edition products that we have republished to date.
If you prefer to use a third-party vendor, we currently have all of our 1st and 2nd editions products for sale at
http://www.wargamevault.com ... and we are very soon going to start putting our 3rd edition products there as well. You can also download our Quick Start Rules and Quick Start Rules for Miniatures for free from wargame vault.
To answer your other questions, each version of Starfire is an improvement that uses the same basic system but has changes. 3rd edition was the version built by David Weber and based on his Starfire novels. 3rd edition is the only version that has supplements for the entire history of Weber's Starfire (aka the Terran Federation history)