To try to shed some light on this question regarding the Panasonic GH series, I have taken a picture of the same scene at the same time of day with the three cameras. I used the same lens, the Lumix G 20mm f/1.7, and I set the aperture to f/1.7. The other settings were: ISO 200, 1/250s, sunny white balance. Here are the out of camera JPEGs:
When isolating only the wall part of the image, and looking at the histograms, it reveals that the ISO ratings are probably a tad bit different:
This indicates that at ISO 200, the Panasonic GH1 appears to be a bit more sensitive, while the GH2 and GH3 appear to be fairly comparable. This is consistent with the DXO ratings, which show the GH1 to be more sensitive than the GH2.
Note that the above results are based on the out of camera JPEG images. I have also developed the RAW images using Lightroom 4.3 RC with no exposure compensation, and gotten these histograms:
These results indicate a similar conclusion as the JPEG tests: The GH1 has the higher sensitivity at ISO 200, while the GH2 and GH3 are fairly similar.
Using the RawDigger program, I extracted the average exposure from the wall area of the images, again from the RAW images. The results are here:
The current version of RawDigger does not set any default black level offset for the GH3, since it is a new camera. For the GH1 and GH2, it sets a black level offset of 15. Some say that the Panasonic GH3 should have a black level offset of 144, in which case you get the rightmost values in the table, denoted by "GH3(*)". The other GH3 column is taken without any black level offset.
For comparison, here are the images converted from the RAW files using Lightroom: