3 Instances can be a lot, and you definitely want to limit memory. It might not be linearly, so not necessarily 1GB per instance, but you do need to leave some for the OS.

The amount each instance needs depends on the load. you'll have to do some testing and maybe adjust the amount of memory (use advice above) across the instances.

The other thing is that you might think about consolidating some of the databases together and removing an instance or two. Is there a reason you have 3 instances?