Your issues:
1) Moire....yes the GH3 definitely does have moire showing up more frequently than the GH2. It's not extreme, but it was worse than the GH2 in that respect. I found it occurred on generally "picket fence" type structures where the GH2 did not have a problem. (I have a photo of a factory with steel ribbed window shutters, and it was very obvious on that), but not much else.
2) No personal problems with lights flickering, but I do not usually shoot in fluorescent lighting situations.
3).Viewfinder: The viewfinder is one area that you will have to look and decide for yourself about when you try the camera. Some people have big problems with it, others have none at all. I was somewhere in the middle.
For me, I noticed the edge blur the first time I would use the camera, then I had no problem with it afterwards. The display is nice, bright, and clear, overall.
There were two viewfinder attributes that were, however, significant enough
for my uses, that it affected my overall happiness with the camera.
These were:
- Viewing area size...since the GH3 uses at 16:9 ratio screen in the EVF, the actual viewing area is smaller than that in the GH2. I prefer the largest EVF viewing screen size I can get, so this seemed a bit like a step backwards for me.
- The OLED EVF does not agree in terms of color rendition with the rear screen, or with the actual colors of the scene. (This is even noted in the owner's manual, so it's not something that Panasonic was unaware of.)
For most people, this is not an issue, as most use the EVF for just framing the scene. However, I take a lot of flower photos where color relationships and WB are very important, and I use the EVF to keep tabs on whether I have everything set right in terms of color and WB as I am shooting. (Yes, you can do that in PP, but it's still easier if you get it right in camera....) As a result, the inaccurate color rendition in the EVF made the camera considerably more difficult for me to use for my purposes.
If you do not have these particular issues with the camera, then the GH3 is going to be a significant step up for you from the GH2.
My suggestion? Buy a GH3, but don't sell your GH2 until you have had a chance to work with the GH3 for a few days. It will either knock you over with how good it is (and it is), or you will find that one of its very few idiosyncracies may lead you back to the GH2.