You should be able to do a left join on to the table itself and calculate this. Since there is no sequential number, you would need to generate one - which is what the CTE in the query below does.
;WITH cte AS
SELECT *,ROW_NUMBER() OVER (PARTITION BY col1 ORDER BY col3) AS RN
a.col1, a.col3, a.col3,
COALESCE(DATEDIFF(dd,b.col3,a.col3),0) AS differenceCol
LEFT JOIN cte b ON
a.RN = b.RN+1
AND a.col1 = b.col1