From BR Bullpen
A three-game playoff is a series of up to three games played after the end of the regular season to determine which of two teams tied with the same record gets to advance to the postseason. While the American League has used a one-game playoff to decide end of the season ties in the standings, the National League used a three-game playoff format before the beginning of divisional play in 1969. Statistics from such playoff games are counted with regular season statistics.
One team plays at home for game 1, and the other for games 2 & 3. The playoff ends as soon as a team has won two games.