DECLARE @myTable TABLE (date datetime, Qty Int)
Declare @intervals int
Declare @myMinDate datetime

Insert @myTable
Select '1-1-2009',8 Union all
Select '1-3-2009',10 Union all
Select '1-8-2009',7 Union all
Select '1-9-2009',15 Union all
Select '1-10-2009',16 Union all
Select '1-11-2009',1


Select * from @myTable

--To Get first date and last date of timeframe you want
Select Min(date) as minDate,max(date) as MaxDate from @myTable

--Using the First and last dates, figure out how many 3 day intervals you need
Select Datediff(d, t.minDate, t.maxDate) as DiffofDays, Ceiling(cast((Datediff(d, t.minDate, t.maxDate)) as float)/3) as IntervalsNeeded from (Select Min(date) as minDate,max(date) as maxDate from @myTable) t

--Set intervals
Set @Intervals = (Select Ceiling(cast((Datediff(d, t.minDate, t.maxDate)) as float)/3) as IntervalsNeeded from (Select Min(date) as minDate,max(date) as maxDate from @myTable) t) - 1
Set @myMinDate = (Select Min(date) as minDate from @myTable)

Select @Intervals
Select @myMinDate

DECLARE @myDateRanges TABLE (date datetime)

--insert the beggining date and then every 3 days after that for as many intervals as required, to set a known date change Set @myDate to a specific day
While @Intervals >= 0 
Begin
Insert @myDateRanges
Select Dateadd(D,3 * @Intervals,@myMinDate)
Set @Intervals = @intervals - 1
End 

Select * from @myDateRanges

Select Convert(varchar(10), b.date ,120) as Date, ISNULL(Sum(case when a.date between b.date and Dateadd(D,2,b.date) then a.qty end),0) as sumQty From @myTable a cross apply @myDateRanges b group by b.date