So, this is a broad question which can have various answers.
Stating the Obvious, with 3 nodes you do not have a single point of failure. Another benefit would be performance and scale since the Nodes can share services between the nodes.
Cisco DNA Center supports a three-node cluster configuration, which provides both software and hardware high availability. A software failure occurs when a service on a node fails. Software high availability involves the ability of the services on the node or nodes to be restarted. For example, if a service fails on one node in a three-node cluster, that service is either restarted on the same node or on one of the other two remaining nodes. A hardware failure occurs when the appliance itself malfunctions or fails. Hardware high availability is enabled by the presence of multiple appliances in a cluster, multiple disk drives within each appliance's RAID configuration, and multiple power supplies. As a result, a failure by one of these components can be tolerated until the faulty component is restored or replaced.
Take a look at:
Cisco DNA Center High Availability Guide, Release 1.3.3.0
https://www.cisco.com/c/en/us/td/docs/cloud-systems-management/network-automation-and-management/dna-center/1-3-3-0/ha_guide/b_cisco_dna_center_ha_guide_1_3_3_0.html
With a Single Node you do have Software HA for the pods running but not Hardware HA. If you backup your data (Automation & Assurance), In most cases you can rebuild a Cisco DNA Center after an RMA and Restore from Backup and make any chnages necessary to recover.
When it comes down to any controller setup that manages your network, how much risk do you want to accept. Depending on your deployment size, priority, and SLAs; you try to mitigate downtime risks.
I hope this helps!
T.
Getting Started with Cisco DNA Center:
https://www.cisco.com/c/en/us/support/cloud-systems-management/dna-center/series.html
