Mirroring content/photos between devices — Tripple the iCloud backup?
No, it doesn't.  iCloud is a syncing solution.  That means it will sync everything to all devices signed-in with the same Apple ID.  

You can read more about setting up iCloud from this Apple support page.

The amount of storage will be same on all your iOS devices.  It might different on macOS because of optimized storage.  It will never be more space (or tripled).