select t.StudentName
     , t.RollId
     , t.ClassId
     , t.marks
     , SubjectId
     , tblsubjects.SubjectName
  from
     ( select sts.StudentName
            , sts.RollId
            , sts.ClassId
            , tr.marks
            , SubjectId
         from tblstudents sts
         join tblresult tr
           on tr.StudentId = sts.StudentId
      ) t
   join tblsubjects
     on tblsubjects.id = t.SubjectId;