Correct. That explains why "Obviously, assigning resources to the VM 'takes them away from' OS X, and so it could cause slow performance of the computer."
What is the specific resource contention scenario you're referring to that would cause a VM assigned 3 cores to perform more poorly than assigning it 2 cores?
For that resource contention scenario to exist, it would mean that there's some process running in OS X that needs 2 cores worth of processing power, and if it doesn't get it, it penalizes other running processes.
For example: at time `t`, this process is scheduled. It needs 2 cores worth of processing power. But VMware Fusion + the VMs running under it are currently consuming 2.001 cores worth of processing power. Because of that, VMware and the VMs are penalized so that they can only consume < 2 cores worth.
That hypothetical scenario is the only way I can conceive of to explain why assigning a VM 3 cores would cause it to perform more poorly than one assigned 2 cores.
I'm technical, so go ahead and lay on me the specifics.