I had similar experience with simple talk.
I tried as follows but at the very least, do not know how to group by username , etc.
SELECT username
,a.move_in_date
,a.Cnt_Lead_id
,Rolling12Months=CASE
WHEN ROW_NUMBER() OVER (ORDER BY a.move_in_date) < 3
THEN NULL
ELSE a.Cnt_Lead_id + b.Cnt_Lead_id
END
FROM [countHistory] a
CROSS APPLY
(
SELECT Cnt_Lead_id=SUM(Cnt_Lead_id)
FROM
(
SELECT TOP 2 username, b.move_in_date, Cnt_Lead_id
FROM [countHistory] b
WHERE b.move_in_date < a.move_in_date
ORDER BY b.move_in_date DESC
) b
) b
ORDER BY a.move_in_date;