Each node has three configuration items: browserName, version and platform. This aThe version is a "free" format. If you give each of your nodes a platformmade up version name (A999,B998 and C997) then you can use the Grid capability to force a test the run on the platformversion name of choice.
Start the nodes with
-browser platform=Aversion=999
In your test ask for a capability:
capability.setPlatformsetVersion("A""999");
If only one node has this platformversion, 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