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?