In 1v1 best of three do we pick our battlefields or are they random?

In a 1v1 best-of-three match, you choose your battlefields.

According to Rule 481.5 and the associated RiftJudge FAQs, the setup process for a best-of-three match is as follows:

• Each player brings three battlefields as part of their deck.
• For each game in the match, players select one of their three battlefields to use.
• Battlefields are revealed simultaneously by both players before determining turn order.
• Once a battlefield has been used in a game, it is removed and cannot be selected again for the remainder of that match. You must choose from the remaining battlefields you set aside.

This differs from a best-of-one duel, where battlefields are selected randomly (Rule 480.5).