foreach ($students as $student_row){ //get each student's ID
 $studentID = $student_row['studentID'];
       foreach ($allExams as $allExam) { //get each exam's ID
       $examID = $allExam['examID'];
            foreach ($subjects as $subject){ //get each subject's ID
               $subjectID = $subject['subjectID'];

$obtained_mark_query = $this->db->query("select * from mark
            where mark.studentID = '$studentID' && mark.examID = '$examID' &&
            mark.subjectID = '$subjectID'")->row()->mark; //to get mark for each student per exam per subject
                        }

              }

}