How do I sum data over 3 month ranges?
You could categorise your dueDate values and then use PIVOT like this:

WITH categorised AS (
  SELECT
    Balance,
    Range = CASE
      WHEN dueDate &lt; DATEADD(MONTH,  3, @Date) THEN '0-3 Months'
      WHEN dueDate &lt; DATEADD(MONTH,  6, @Date) THEN '3-6 Months'
      WHEN dueDate &lt; DATEADD(MONTH, 12, @Date) THEN '6-12 Months'
      ELSE '&gt;12 Months'
    END
  FROM
    dbo.YourTable
  WHERE
    Company = @Company
    AND dueDate &gt;= @Date
)
SELECT
  [0-3 Months],
  [3-6 Months],
  [6-12 Months],
  [&gt;12 Months]
FROM
  categorised
PIVOT
  (
    SUM(Balance)
    FOR Range IN ([0-3 Months], [3-6 Months], [6-12 Months], [&gt;12 Months])
  ) AS p
;