Develop module, Lightroom menu:
Ensure there is a checkmark for all the panels that you want displayed, including tone curve.
Alternatively, press Ctrl/2.
The images from LR 3 were processed using a previous Process Version that did not include RGB Tone Curves. In order to use the RGB tone curves, you would have to upgrade the photo's Process Version to Version 2012. Use Settings->Process->2012. Please be aware that if you do this, it is likely the appearance of your photos will change and you may have to re-edit the photo.
Yes, this was the answer I was looking for. When I check the Settings->Process the old image was marked as 2010, not 2012. As soon as I change the process to 2012 I was able to get at the Tone Curve RGB options.