SQL Clusters require a unique virtual name for each instance so you cannot do:

SQLName\Instance1

SQLName\Instance2

SQLName\Instance3

You must do:

SQLName1\Instance1

SQLName2\Instance2

SQLName3\Instance3

I believe this is because the virtual name/IP can only be active on a single node at a time and therefor if you have 3 instances each could be active on a different node.

CEWII