Något i stil med:
SELECT topsettlunit,
settleunit,
CONNECT_BY_ROOT( topsettlunit ) AS level1,
CASE LEVEL
WHEN 1 THEN settleunit
WHEN 2 THEN topsettlunit
END AS level2,
CASE LEVEL
WHEN 2 THEN settleunit
END AS level3
FROM your_table
START WITH topsettlunit NOT IN ( SELECT settleunit FROM your_table )
CONNECT BY PRIOR settleunit = topsettlunit;