AlwaysOn Availability Groups: connecting correctly?
There's some weirdness when you use AG Listeners along with multiple standalone SQL instances.  Note that the more common configuration is to install a single standalone instance per node, possibly participating in multiple AGs.

When you enable TCP/IP connections for a SQL instance, the default configuration is for the SQL instance to listen on a port (either fixed or dynamic) on "All IP Addresses":



For standalone instances "Listen All" means that it will listen for incoming connections on all current and future IP addresses.  If an IP address is later added to the system, the instance is already listening on its port on the new IP.  And an AG Listener is just a Network Name and IP that the cluster adds to whichever node hosts the primary replica.

For Failover Cluster Instances, "Listen All" means that the instance will listen on all IP addresses that are dependencies of the FCI's Virtual Network Name, so they don't have this problem.

So, when you have multiple standalone instances where one or more participates in an AG with an AG Listener, you need to perform some extra configuration to prevent every instance from listening on every AG Listener's IP. 

Configure each instance to listen on a non-default port (can be dynamic), only on the host's main IP(s), reserving port 1433 for the AG Listener(s).  That way you can connect to each AG Listener without a port number or instance name, and to the instances directly only with a port number or instance name.