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;