El Capitan download for Mac Pro - App store "system is too new"
You'll struggle... the 3,1 won't boot from USB.
People say you can but I've had one since new &amp; no amount of cajoling has ever made it do so. 

If you have El Capitan in your purchase history on the Mini, then that's where you will be able to download it. The version linked in the App Store can only be run on machines that cannot run Sierra.

If you have a clean external USB drive, you could get the installer onto it the same way as you would have onto a USB stick. If you then drop the drive in the 3,1 it will boot from that.
Alternatively, once you have the right installer, you can run it to the external from the Mini &amp; then swap it into the 3,1.