After the radius servers are defined, we will group them under a group name so that its easy to reference all 3 servers with just one name. It's not just about the name, but it also means that we have a backup radius server. If one fails, we have other radius servers that could be used for authentication.

ISR-LAB(config)#aaa group server radius rad-group
server name rad01
server name rad02
server name rad03