mac pro 3.1 OS X version

The Classic Mac Pro 3,1 will support OS X 10.5.1 to macOS 10.11 El Capitan

you will need OS 10.6.8 installed or higher in order to install El Capitan.

Only El Capitan is available to download from that computer using the App store if you do not have any other version of the OS physically on hand

OS 10.6.8 still available from Apple for purchase

this is the link from Apple but it no longer goes to the 10.6.8 page

https://www.apple.com › shop › product › mac-os-x-106-snow-leopard

to download El Capitan see this link

https://support.apple.com/en-us/HT206886

OS X El Capitan will run on the MacBook 

Version 10.7.5