The issue that causes "Error MSB3073 exited with code 3" happens to be with the VALUE in the Outputs property under Project > Properties > Configuration Properties > Custom Build Step.
When I choose Edit for Outputs, a dialog box comes up. I enter a value of "$(OutDir)\regsvr32.trg" in the upper textbox then click OK. Now for some reason VS 2010 changes the display text to show "$(OutDir)\regsvr32.trg;%(Outputs)". I'm not sure why or if that is the cause of the problem.
However, I will get "Build Successful" if I change my Output value to "<inherit from parent or project defaults>" instead of choosing Edit and putting a value in. No more Error MSB3073 exited with code 3.
I guess I can say this issue is resolved but I'd like to know why/what causes this issue to begin with.