Who decides first/second after game one?

In a best-of-3 match, the loser of the previous game decides who plays first or last in the next game.

According to Tournament Rule 407.4:

"For games after the first game of a match, the loser of the previous game gets to choose if they play first or last. If the previous game was a draw, the starting play from the previous game is maintained."

Additionally, per RiftJudge FAQ 3308, this decision is made after the battlefield for the next game has been chosen.