Thought I'd make a new thread for this issue.
For the 'Crossfire Flickering Fix!!!' go here Witcher 3 Crossfire Flickering Fix!!!
So...I'll order the fixes in order of most likely cause/relevance, and a short description below it.
Below that I'll put my settings, these are just suggestions, specifically for a 280x (that's my card).
For any change in 'user.settings' file to take effect you must 1st delete 'user.settings.bak', do this every time you make a change.
I would also suggest making a backup of this file, it is located in your 'My Documents/Witcher 3' folder.
1.Turn on the mouse cursor setting in graphics menu.
(Some sort of mouse smoothing setting)
2.Lower the 'Character LOD distance' in user.settings under 'Rendering/TextureStreamingCharacterDistanceLimit='
(This is the distance at which the higher resolution texture files for characters get rendered)
(Mine is set at 25, and I'm not getting any pop-in with this setting)
3.Lower the 'General LOD distance' in user.settings under 'Rendering/TextureStreamingDistanceLimit='
(This is the distance at which the general textures like walls fences etc. get rendered)
(Mine is set at 20000, don't go too low for this one or you will start to notice pop-in)
4.Lower the 'Character Heads LOD distance' in user.settings under 'Rendering/TextureStreamingHeadsDistanceLimit='
(This is the distance at which the higher resolution texture files for character's heads get rendered)
(Mine is set at 5, again no pop-in with this setting)
5.Raise the 'Texture Memory Budget' in user.settings under 'Rendering/TextureMemoryBudget='
(This is the amount of v-ram the game assigns for texture handling)
(Mine is set to 1014, but be very careful with this as a higher value than your card can handle will hard-crash your pc, 1014 is fine for 3gb cards.)
(But should you experience instability it's probably this setting, if that's the case just revert to the default value of 800)
6.'Flip Queue Size' in RadeonPro under Advanced just tick the box and change it to a higher value (0-5).
You need to create profile for witcher3.exe 1st, then right-click and 'Disable API Monitoring'.
(This determines how many frames the GPU renders ahead of displaying it)
(My setting is at 2, if you go too high (e.g. 5) you might get input lag (slower reaction to button presses and mouse movement))
7.Make the 'witcher3.exe' run at high priority. Run The Witcher 3, when at the menu ctrl-alt-delete and Open 'Task Manager' under 'Processes' right-click witcher3.exe 'Set Priority/High'.
(This determines which process is more important than others currently running)
(I set mine at 'High', but avoid the 'Realtime' setting...this will cause instability)
If your GPU can handle it you can also force FXAA in RadeonPro, for those that want more eye candy :-).
You don't need to try them all if you don't want to, 5-6 and 7 aren't that important but do make an overall difference.
It's also generally a good idea to always run a game as administrator, and take ownership of the entire Witcher 3 install directory.
I've tested this with and without Crossfire on a 280x.
If you're still having problems please describe the problem as accurately as possible, include CPU/GPU info, and I will try to help.