There is NO stage3-amd64 inside releases/amd64/autobuilds/current-stage3-amd64 directory in must German mirrior, but current-stage3-amd64-hardened. I checked: Netcologne (http)* Uni Erlangen-Nürnberg (http)*
I believe was due to someone accidentally deleting those images. We built new ones, apologies for the trouble! http://mirror.netcologne.de/gentoo/releases/amd64/autobuilds/current-stage3-amd64/ (for example.)