Hello Bodefeld.

You can do assigning core quite easy by starting 'taskmanager' (on finnish version atm so not certain about name here) and rightclicking on wow.exe to set affinity.
Then proceed to remove all checks but one (1st cpu).

Repeat this for every instance of wow.exe, just check 2nd cpu on second instance and 3rd cpu on 3rd etc.

If you have seperate folders for all WoWs you can do this by editing wows config file too.