Basically a 3 node PostgreSQL Cluster, running 3 processes i.e. PostgreSQL DB, PGPool and WatchDog.
These 3 nodes will be distributed across 3 availability zones/data centers for resilience and use a synchronous replication between Primary and Stand-by.
You're describing HA+DR, not just HA,
Also, I wouldn't do synchronous replication across the WAN. Not only is the latency too high for decent performance, but any fault in the network freezes the DB.
Synchronous option will be Any One, so that the DB availability is not impacted if 1 Stand-by is down for even planned outage i.e. Patching of DB or Virtual Machine.
You can switch from async to sync replication just before patching, and then switch back to async when it's completed.
That's pretty much what we do for HA, except only two DB instances (but still three PgPool instances), and they are local and asynchronously replicated. DR is handled by VMware SRM.
Watchdog and heartbeat are built into PgPool. Is that what you're using for WD and HB?