Can you clarify the timeframe for the activations? Clearly, using the same LiveID to activate all 3 computers should work fine unless the first 2 are running the beta code installed and activated with a beta key prior to November's release of 2.0 and the x64 3rd PC is running the released code.
Can you also verify in the first two PCs that you are indeed using the same LiveID - open OneCare on both PCs and click Help/About. The Subscription ID is the LiveID you would need to use on the 3rd activation.
-steve