How much of Xfce4 is GTK3?

There is no such list - and even if one existed, it would be outdated fairly quickly.

This is the case even if you're only counting "official" xfce apps, ignoring the thousands more GTK2 & GTK3 programs that can be run on a system running xfce.

The important thing to know is that, regardless of whether you use xfce or not, if you run GTK apps you will need a theme compatible with both GTK2 and GTK3 installed. This will be the case for years to come - don't expect this to change any time soon.