In the context of driver compatibility, Cassandra "3.0+" means C* 3.0 and newer releases which include C* 3.0.x, 3.11.x, 4.0.x and [soon] 4.1.x.
To answer your question directly, version 2.6 of the C++ driver works with C* 3.11.11 but we don't recommend you use it since it's an ancient release that's over 5 years old. If you run into issues, the first response you'll get is to upgrade. Instead, we recommend using the latest release of the drivers if you're about to start a new project. Cheers!
