We were asked this a few times, so I think it is worth clarifying.
If you have a subscription license to RavenDB, you have automatic access to all versions of RavenDB for as long as your subscription is current. That means that if you purchase a RavenDB 2.x subscription, your license allows you to use RavenDB 3.0 without any issues.
Note that this doesn’t include using RavenFS, which will require an updated license.
If you purhcased RavenDB using the one time code, you’ll need to purchase a new license for RavenDB 3.0.