(1)Report Builder 3.0 : is it a production release ? Or just a beta version ?
It is in beta because it comes with SQL Server 2008 R2.
(2)Is it free (of course, I assume one has already bought SQL Server) ?
It is free if you are running SQL Server 2008 R2.
(3)I am just using SQL Server 2008 Enterprise Evaluation Edition, can I use it with Report Builder 3.0 ?
No SQL Server 2008 comes with Report Builder 2.0.