HI Charlie,
Yes - you can install 3.0.x (3.1.5 was never a program version, but the database version that correlates to 3.0.x).
You will find a free program to activate your v3.0 license at the bottom of the Product Info >> Upgrading from 2.x or 3.0 page on this site. You must be logged on to this site to see it.
Also, we no longer have that version for download on this site, so you must have the CDrom for it. Also, you may only have the CDrom for an earlier version of 3.0.x. In that case, install it from the CDrom, restore your database from a backup and see if it works. If it complains about being the wrong version, there is also a free upgrade to the version you will need at the bottom of the same page mentioned above.
Let me know if I can assist further,
Jeff |