For some reason, the GTK3 apps I use are shown with DejaVu Sans Book instead of DejaVu Sans Condensed, which is the one I set for the GTK2 apps. At first I thought this was just a settings issue, but then I switched to another font and both the GTK2 apps and the GTK3 apps used the same font. But when I set DejaVu Sans Condensed, the GTK3 apps use DejaVu Sans Book instead.
But when I change the font to DejaVu Sans Oblique, both apps use the same font:
This is so weird... is there any way to make sure GTK3 apps use DejaVu Sans Condensed?