SELECT wr_7, wr_5, wr_subject, wr_9 FROM g5_write_sub_01WHERE wr_datetime > (NOW() - INTERVAL 3 DAY
GROUP BY wr_7, wr_5, wr_subject, wr_9
group by는 where 뒤에 있어야 합니다.
https://dev.mysql.com/doc/refman/8.0/en/select.html
$sql = " SELECT wr_7, wr_5, wr_subject, wr_9 FROM g5_write_exam_location WHERE wr_datetime > NOW() - INTERVAL 3 DAY GROUP BY wr_7, wr_5, wr_subject, wr_9" ;