DECLARE @xmlIN XML = '<TableData>
<aaa Main="First">
  <row name="a" value="1" />
  <row name="b" value="2" />
  <row name="c" value="3" />
</aaa>
<aaa Main="Second">
  <row name="a" value="3" />
  <row name="b" value="4" />
  <row name="c" value="5" />
</aaa>
<aaa Main="Third">
  <row name="a" value="10" />
  <row name="b" value="20" />
  <row name="c" value="30" />
</aaa>
</TableData>'

SELECT t.col.value('../@Main', 'varchar(10)') [Header],
t.col.value('@name', 'VARCHAR(25)') [name],  
t.col.value('@value',  'VARCHAR(25)') [Value]
FROM    @xmlIn.nodes('//TableData/aaa/row') AS t (col)

Header    name    Value
First      a        1
First      b        2
First      c        3
Second     a        3
Second     b        4
Second     c        5
Third      a        10
Third      b        20
Third      c        30