I'm guessing .375brno may already have a .375 .

I'd keep it. But I am also a hoarder.

All three are great CLASSIC VINTAGE cartridges. The .30-06 may not seem like it because it is so popular, but there is a reason it is popular, because it is a good cartridge.

I would keep it as ammo and components are easy to get.

Otherwise as suggested, stock up big time with components for the .275 and especially the .318.

Australia does not yet have any real limitations in gun numbers you can own. In the meantime, a good reason to keep them. IMO.