Give this a try...

Sub ShowOutlineLevel()

Const SUMMARY_LEVEL = 1
Const SECOND_LEVEL = 2
Const THIRD_LEVEL = 3

'ActiveSheet.Outline.ShowLevels RowLevels:=SUMMARY_LEVEL
'ActiveSheet.Outline.ShowLevels RowLevels:=SECOND_LEVEL
ActiveSheet.Outline.ShowLevels RowLevels:=THIRD_LEVEL

End Sub

Thanks..it works.
But I found that it's very strange that now I need to click the original level
1, 2 ,3 button provided by Excel in the left side of the spreadsheet "twice" to
unfold/fold (it used be to "once"). Why does this happen?

Please try a very simple case and you would duplicate my problem right away.

Not sure what's going on, but I couldn't replicate your problem - even
tried it upto 6 levels.

I found that because I placed the code  in a command box, then when I
press the command box to display only level1, it worked as
expected. At this time, I lost the cursor. Then if I want to click
the original level  1, 2 ,3 button provided by Excel in the left side
of the spreadsheet, I need to do it twice: first time to bring
cursor's attention back to spreadsheet, second time to do the normal
fold/unfold job. I sloved this problem by adding something like
--
Cells(currentRow, currentCol).Select
--
at the very end of the code in command box to bring cursor's
attention back. It is OK now.

But, I just wonder must I do this extra step? Shouldn't Excel do
that for us?