Quoted:
The new panel is a sub-panel and you can only use a 3-wire connection if there are NO other metallic connections between the buildings (like water line, phone lines, cable, etc.)
If it is as shown above, it is not a sub panel. It would be the first point of service, a sub panel would be after that panel.
I believe OP is building a rack type service that is out in the open and free standing of any structures, similar to this 400 amp service for a pole barn:
The grounds and neutrals are bonded at this point, but not after.