For a cost-effective high availability deployment in a campus enterprise wireless network with three controllers, the N+1 architecture with Stateful Switch Over (SSO) is recommended. This architecture provides redundancy and allows for seamless failover without service interruption, ensuring that the network remains highly available without the need for a one-to-one controller redundancy, which would be less cost-effective
https://www.cisco.com/c/en/us/td/docs/wireless/technology/hi_avail/N1_High_Availability_Deployment_Guide/