The number of nodes required by the consistency level is a function of the replication factor, not the number of nodes in the cluster. So for RF=2, a quorum is 2 so all your nodes must be up to be able to read all your data.
With 3 nodes in your cluster, RF=2 and reading at CL.QUORUM, you can only access 1/3 of your data with one node down. Reads for other keys will result in an unavailable exception.
With 3 nodes in your cluster, RF=3 and reading at CL.QUORUM, you can still access all your data with one node down.