Actually still in Gnome 3... you just need to use the "advanced settings" tool.

[I don't agree with all their choices of what to call an "advanced" setting, but I think it's appropriate classification in this case.]

I know, and I'm using it for this, but the previous dialog was just way better and user-friendly. It had large previews and descriptions while the new tweak tool only has some uncommented options in a drop-down menu.

Gnome Tweak Tool still allows you to do this, among many other things. I consider it an essential part of the Gnome 3 desktop.

You can still edit ~/.fonts.conf by hand. If you need an intermediate level, then you're advanced enough to use vim and write the config yourself. The GUI shouldn't have to pander to fickle powerusers like you, it's supposed to make the PC EASY.