Write the syntax to access third level record field using object oriented peoplecode?

If you know the row number in advance use below. Note I have put row number as 1 for all the levels.

&fld=Getlevel0()(1).GetRowset(Record.<level1 record>.GetRow(1), 

                                 GetRowset(Record.<level2record>).GetRow(1),

                                  GetRowset(Record.<level3 record>).GetRow(1),

                                  GetRecord(Record.<level3 record>).GetFieild(Field.<field name>));

To loop through and find the field use below code.

&rsLvl0 = GetLevel0();

For &I = 1 to &rsLvl0.ActiveRowCount

 &rsLvl1 = &rsLvl0.GetRow(&I).GetRowset(Scroll.LEVEL1_REC);

    For &J = 1 To &rsLvl1.ActiveRowCount

      &rsLvl2 = &rsLvl1.GetRow(&J).GetRowset(Scroll.LEVEL2_REC);

        For &K= 1 To &rsLvl2.ActiveRowCount

          &rsLvl3 = &rsLvl2.GetRow(&K).GetRowSet(Scroll.LEVEL3_REC);

            For &L = 1 To &rsLvl3.ActiveRowCount

              &Rec3 = &rsLvl3.GetRow(&L).GetRecord(Record.LEVEL3_REC);

              &Field = &Rec3.FIELD_NAME;

              rem Now do your processing;

           End-For;

      End-For;

   End-For;

End-For;