I assume that you've tried turning off "Incremental rendering" or increasing the LOD (Level of detail) to maximum? I had a problem like you are describing and that fixed it for me.
Where do you access the graphics? The flickering is so bad I have to guess where anything on the screen is, but so far I haven't been able to find any options.
I installed the game again to be sure, but if you cannot see anything to adjust the graphics in-game, then you could try editing the entry for Gabriel Knight 3 in the Windows Registry. For some reason, they chose to place those settings there and not in a .ini or .cfg type of text file. If you need help on how to use the registry, feel free to ask, or check out how via a Google Search. Just be careful not to touch anything other than the stuff mentioned.
The options should be located at: Computer\HKEY_CURRENT_USER\Software\Sierra On-Line\Gabriel Knight 3\Engine\Hardware
Inside I found the following options shown in the screenshot I have attached. You can adjust the "Incremental Rendering" to "0" by double-clicking on it, changing the number to 0, then pressing ENTER or clicking OK. This is the option that stopped the flickering for me.
If you can see the game from that point, you can adjust the graphics options in-game by right-clicking somewhere and clicking on the options button.
If you want a resolution other than the ones the game provides for you, you can adjust this in the registry under:
Computer\HKEY_CURRENT_USER\Software\Sierra On-Line\Gabriel Knight 3\Engine
There should be a "Screen_Height" and a "Screen_Width" option which you can change to the resolution of your choice.
I hope this is of help to you. I'm no expert on this sort of thing, but I have had to fiddle with it in the past.