There is a simpler way.

No updates required:

--===== Create a test table.

-- This is NOT part of the solution.

IF OBJECT_ID('TempDB..#ST') IS NOT NULL DROP TABLE #ST

DECLARE @N int

SET @N = 3 -- how many rows to retrieve for each group

--create the temp table, with Identity column playing the role of RowNumber

select

identity(int,1,1) AS ID,

territoryID, salesPersonID, SalesTotal

INTO #ST

from (select

TerritoryID,

salesPersonID,

sum(TotalDue) as salesTotal

from [Sales].[SalesOrderHeader]

where SalesPersonID is not null

group by TerritoryID, SalesPersonID ) ST

ORDER BY TerritoryID,

salesTotal DESC -- arranging ID's within the each group

SELECT T.*

FROM #ST T

INNER JOIN (

SELECT TerritoryID,

MIN(ID) FromID, MIN(ID) + @N ToID

--MIN(ID) is a starting point for rowcount within each group

FROM #ST

GROUP BY TerritoryID

) DT ON DT.TerritoryID = T.TerritoryID

AND T.ID BETWEEN DT.FromID AND DT.ToID

ORDER BY T.TerritoryID, T.ID