I'm not sure what you intend to use these results for, most cases would want some sort of relationship defined, but to replicated your stated required output you could use:

DECLARE @TABLE TABLE (lvl1 NVARCHAR(50), lvl2 NVARCHAR(50), lvl3 NVARCHAR(50));
INSERT INTO @TABLE (lvl1, lvl2, lvl3) VALUES
('Oracle', 'Marketing unit', 'Internet           '), ('Oracle', 'Lawyers unit  ', 'Intellectual       '), 
('Oracle', 'Finance unit  ', NULL),                  ('Oracle', 'Lawyers unit  ', 'Judicial department'), 
('Oracle', 'IT unit       ', 'Database           '), ('Oracle', 'Marketing unit', 'Television         '), 
('Oracle', 'IT unit       ', 'ERP                '); 

;WITH base AS (
SELECT DISTINCT lvl1, '' AS lvl2, '' AS lvl3
  FROM @TABLE
UNION ALL
SELECT DISTINCT lvl1, lvl2, ''
  FROM @TABLE
UNION ALL
SELECT DISTINCT lvl1, lvl2, lvl3
  FROM @TABLE
 WHERE lvl3 IS NOT NULL
)

SELECT CONCAT(CASE WHEN LAG(lvl1,1) OVER (ORDER BY lvl1, lvl2, lvl3) IS NULL THEN lvl1 WHEN LAG(lvl1,1) OVER (ORDER BY lvl1, lvl2, lvl3) = lvl1 THEN CHAR(9)+' - ' ELSE lvl1 END,
       CASE WHEN LAG(lvl2,1) OVER (ORDER BY lvl1, lvl2, lvl3) IS NULL THEN lvl2 WHEN LAG(lvl2,1) OVER (ORDER BY lvl1, lvl2, lvl3) = lvl2 THEN CHAR(9)+' - ' ELSE lvl2 END,
       lvl3)
  FROM base
 ORDER BY lvl1, lvl2, lvl3

All we're doing here is making seperate rows for the groups, and then sticking them back together, while comparing this row to the last row using LAG() .

Your structure is a little clumsy, but this would be easy to expand up to 10 levels

You may notice the siblings are in alphabetical order, if you wanted a specific order, there would need to be another column with the proper order/sequence.

Example or dbFiddle

DECLARE @YourTable TABLE (lvl1 VARCHAR(50), lvl2 VARCHAR(50), lvl3 VARCHAR(50));
INSERT INTO @YourTable (lvl1, lvl2, lvl3) VALUES
('Oracle', 'Marketing unit', 'Internet'), ('Oracle', 'Lawyers unit  ', 'Intellectual       '), 
('Oracle', 'Finance unit', NULL),                  ('Oracle', 'Lawyers unit  ', 'Judicial department'), 
('Oracle', 'IT unit', 'Database'), ('Oracle', 'Marketing unit', 'Television         '), 
('Oracle', 'IT unit', 'ERP'); 

;with cte0 as ( Select Distinct Pt=Lvl1+null,ID=Lvl1 From @YourTable Where Lvl1 is not null )
     ,cte1 as ( Select Distinct Pt=Lvl1     ,ID=Lvl2 From @YourTable Where Lvl2 is not null )
     ,cte2 as ( Select Distinct Pt=Lvl2     ,ID=Lvl3 From @YourTable Where Lvl3 is not null )
     ,cteC as (
                Select * from cte0
                Union All
                Select * from cte1
                Union All
                Select * from cte2
              ),
     cteOH as (
     Select *,Lvl=1,SPath=convert(varchar(500),ID) From cteC Where Pt is null
     Union All
     Select h.Pt,h.ID,Lvl=cteOH.Lvl+1,sPath=convert(varchar(500),cteOH.sPath+'/'+h.ID) FROM cteC H join cteOH ON h.Pt = cteOH.ID
    )
    Select Lvl
          ,ID
          ,Pt
          ,Nested = replicate('|---',Lvl-1)+ID
     From cteOH
     Order By sPath

Results

I think there may be a simpler way, unless I am missing the point. Why not simply group and sort? You can do a series of these with "go" in the middle, or just one big table:

SELECT lvl1, lvl2, lvl3, ... lvn
FROM Table
ORDER BY lvl2, lvl3,....lvn;

Or do a summary queries of each level

select lvl2,lvl3
from Table
order by lvl3
go
select lvl3,lvl4
from Table
order by lvl4
go
...
select lv(n-1),n
from Table
order by n

But I might be missing the point, because this is simple. I just wanted to include it because someone gave me a simple answer once, and it was the correct one.