SELECT mrmatter,
ttk,
SUM(D1Rate) AS D1Rate,
SUM(D2Rate) AS D2Rate,
SUM(D3Rate) AS D3Rate 
FROM
(SELECT

D1.mrmatter
, D1.ttk
, D1.D1Rate
, NULL AS 'D2Rate'
, NULL AS 'D3Rate'

From D1

UNION ALL

SELECT 

D2.mrmatter
, D2.ttk
, NULL AS 'D1Rate'
, D2.D2Rate
, NULL AS 'D3Rate'

FROM D2

UNION ALL

SELECT

D3.mrmatter
, D3.ttk
, NULL AS 'D1Rate'
, NULL AS 'D2Rate'
, D3.D3Rate

FROM D3
)t
GROUP BY mrmatter,ttk