The binary package is obviously available across mirrors: http://download-ib01.fedoraproject.org/pub/fedora-secondary/updates/26/i386/Packages/p/python3-3.6.4-1.fc26.i686.rpm
The repodata, however, does not contain this python3 package. Only source RPM references to it are available for some binary packages built off this source package.
This can be seen by, e.g., looking at the generated repodata manually: http://download-ib01.fedoraproject.org/pub/fedora-secondary/updates/26/i386/repodata/44bd0d9e709ea8aa8101c248b0bda32a3b7d7d666704568a9ffefb74ebf31f1a-primary.xml.gz
This issue, as far as I can tell, only affects the i386 repository. x86_64 is fine.
Fixed, likely by a metadata update - at least for this package. Haven't checked all the others.
Metadata Update from @ionic: - Issue close_status updated to: Fixed - Issue status updated to: Closed (was: Open)