What happens to battlefield cards that are not selected during setup - can they be placed back in the deckbox with the sidedeck or must they remain visible?

Ruling: Un-selected battlefield cards can be placed back in your deckbox. You only need to ensure you have three total battlefields.

Nuances:

• The un-selected battlefields can be stored with your sidedeck
• Only the total count of three battlefields matters for deck legality