Multiple instances are used not for increase overall performance, but for increase manageability.

One instance or three all share the same physical resources.

In my opinion one instance is better from performance point of view because SQL can optimize resource usage across all databases, while in 3 instances servers does not communicate with each other but compete for memory, CPU and disks.

You can use separate instances if you want to increase performance of one or few specific databases.

You may want to guarantee some memory and performance advantage to some particular databases. Then run this specific databases in separate instance and use minimum server memory for that instance. You can also make some CPUs available to one instance while not available to the rest.

Regards,

Slawek