1998 – At Veterans Stadium, Mark McGwire of the St. Louis Cardinals hits three two-run home runs in a 10 – 8 victory over the Philadelphia Phillies. It is the second time this season and fourth time in his career that McGwire has hit three home runs in a game. He also becomes the 12th player in major league history to have two three-home run games in a season.

On May 18, 1998 — 1998 – At Veterans Stadium, Mark McGwire of the St. Louis Cardinals hits three two-run home runs in a 10 – 8 victory over the Philadelphia Phillies. It is the second time this season and fourth time in his career that McGwire has hit three home runs in a game. He also becomes the 12th player in major league history to have two three-home run games in a season.

Source:
Baseball Reference May 18