Currently I have found that Debian only offers six GTK3 themes.
For what it's worth, there are quite a few themes here: https://www.gnome-look.org/browse/cat/135/
My favorite theme, the only one that does not assault my eyes, is not available in GTK3 through Debian's package system.
If you mean it's available for GTK3 (outside of Debian repos), you could just install it manually. You can install into `~/.themes` if you don't want to mess with package manager/system wide stuff.