Macpro 3.1 won't boot from USB
When that model was released, the mainstream boot support was FireWire and APT and not GUID format. If you can’t try that, you’ll want to get a USB to sata controller ($5 to $10) and write the data to a sata drive and connect that bus. 

You should be able to USB boot on Intel Macs, though and the only articles I can find show 10.4.5 adding boot support, so I think you might also just have a hardware issue.


Mac pro early 2008 won&#180;t boot from pendrive with yosemite install app
https://support.apple.com/kb/SP11?locale=en_US
https://www.engadget.com/2008/12/18/mac-101-yes-intel-macs-can-be-booted-from-an-external-usb-drive/