You can always do it the old-fashioned way:
;WITH s AS (SELECT SUM(((Hours * 60) + Minutes) * 60 + Seconds) AS t FROM myTable)
SELECT CAST(t / 60 / 60 AS VARCHAR(10)) + ':' + RIGHT('0' + CAST((t / 60) % 60 AS VARCHAR(2)),2) + ':' + RIGHT('0' + CAST(t % 60 AS VARCHAR(2)), 2) AS total
FROM s