If you look onto their website it shows that the sensor can do multiple aspect ratios. Also the EVF can display 16:9.
Most mirrorless cameras can do that, but what was special with the GH1/2 was that the 16 : 9 and 3 : 2 aspect ratios had more pixels, longest side, so that they were still 16MP (12MP for the GH1).
Here's the data from the Panasonic homepage for the GH3:
[4:3] 4,608 x 3,456 (L), 3,264 x 2,448 (M), 2,336 x 1,752 (S), 1,824 x 1,368 (When attaching 3D lens in Micro Four Thirds System standard)
[3:2] 4,608 x 3,072 (L), 3,264 x 2,176 (M), 2,336 x 1,560 (S), 1,824 x 1,216 (When attaching 3D lens in Micro Four Thirds System standard)
[16:9] 4,608 x 2,592 (L), 3,264 x 1,840 (M), 1,920 x 1,080 (S), 1,824 x 1,024 (When attaching 3D lens in Micro Four Thirds System standard)
[1:1] 3,456 x 3,456 (L), 2,448 x 2,448 (M), 1,744 x 1,744 (S), 1,712 x 1,712 (When attaching 3D lens in Micro Four Thirds System standard)
Ah... dpr seems to have removed the paragraph about the multiaspect sensor now.