Each node has three configuration items: browserName, version and platform. The version is a "free" format. If you give each of your nodes a made up version name (999,998 and 997) then you can use the Grid capability to force a test the run on the version name of choice.

Start the nodes with

     -browser version=999

In your test ask for a capability:

    capability.setVersion("999");  

If only one node has this version, the test should start the run on that node.

For more info about Grid configration look at: https://code.google.com/p/selenium/wiki/Grid2