How to build an intranet nexus yum proxy to external network neuxus yum proxy

pc1 has a nexus yum proxy
pc2 has a nexus yum proxy to pc1
pc3 yum config direction pc2

error info:
failure: repodata/repomd.xml from extras: [Errno 256] No more mirrors to try. [Errno 14] HTTP Error 404 - Not Found