I think this will do it; performance will depend on the indexes available on the table:

select sales.*

from (

select distinct TerritoryID

from [Sales].[SalesOrderHeader]

) AS terrs

inner join

(select top 3

TerritoryID,

salesPersonID,

sum(TotalDue) as salesTotal

from [Sales].[SalesOrderHeader]

where

TerritoryID = terrs.TerritoryID and

SalesPersonID is not null

group by TerritoryID, SalesPersonID

order by sum(TotalDue) DESC

) AS sales on

sales.TerrorityID = terrs.TerritoryID