Thanks,
I'm on Debian testing and have just managed to compile wxWindows 3.0.2 against GTK 3.18, then pgadmin3 against that.
I'm surprised to report that it compiles and runs fine! I'm impressed by the API compatibility of wxWindows.
However, my initial reason for exploring this was that the standard pgadmin3 compiled against GTK 2 wasn't themed to my GNOME 3 desktop which I thought would be resolved by using GTK 3 but it does not. Some elements works as expected, like for instance the GTK 3 file browser is opened, but other issues remain. The most glaring example is that the browser tree items have a white background regardless of theme (applies to both GTK 2 and 3), and will be unreadable if using dark themes (that have light colored text).
/Björn