Driver={OpenLink Virtuoso};Host=server1:port1,server2:port2,server3:port3;UID=dba;PWD=dba;
Driver={OpenLink Virtuoso};Host=server1:port1,server2:port2,server3:port3;UID=dba;PWD=dba;RoundRobin=Yes"

Or alternatively ensure the "use Round Robin for failover connection" check box in the setup dialog.

Figure 7.35. ODBC Round Robin Connect String format

ODBC Round Robin Connect String format