How should the architect recommend the NetScalers be implemented to increase site resiliency?

Global Server Load Balancing (GSLB) is the most effective feature for enhancing site resiliency in a multi-datacenter environment. GSLB intelligently monitors the health of services across different datacenters and dynamically directs user requests to the optimal or available site, ensuring continuous service access even during a datacenter-wide outage.

Creating a single cluster instance encompassing all three appliances across geographically distant datacenters is not a viable solution for site resiliency; clusters require low-latency, high-bandwidth interconnects and are not designed for distributed operation over WAN links.

Configuring a backup vServer primarily provides redundancy for a service within a single NetScaler instance or datacenter, which does not address overall site resiliency across multiple, geographically separated datacenters.

While NetScaler appliances support failover within a high availability pair, configuring 'failover sites within the NetScaler' is not a specific feature for inter-site resiliency; site-to-site failover and traffic redirection are fundamentally handled by GSLB.