SQL Server Failover Cluster Scalable by adding nodes? In the book Pro SQL Server Always On Availability Groups They write about scalability. Here is what they say 


  Windows Server 2012 (and above), failover clustering supports up to 64
  cluster nodes. This helps to easily scale out the solution by adding
  nodes to it. For example, if you have a two-node failover cluster
  hosting applications that is getting close to hitting a node-specific
  capacity limit such as a CPU or memory, it is very easy to add a third
  node and redistribute the load.


How is that possible? If you have one instance on two nodes and you need to add memory. How will another node help? You can't load balance to 3 nodes. I completely understand running two instances ( one on each node) but then adding a 3rd will provide what?
It doesn't mean that it provides every type of scalability, such as for adding memory and CPUs to a given instance.  It means that you can scale up the overall environment by installing more instances that are spread across various nodes and have different possible owning nodes.