Sql-to-linq Group By and Sum Problem

How can I make a linq for this query

SELECT     SUM(CASE Positive
                            WHEN 0 THEN HowMuch
                            WHEN 1 THEN HowMuch * (- 1)
                          END)
                     AS Positive
FROM         dbo.Table1 INNER JOIN
                      dbo.Table2 ON dbo.Table1.Table1ID = dbo.Table2.Table1ID INNER JOIN
                      dbo.Table3 ON dbo.Table2.Table2ID = dbo.Table3.Table2ID

var query = from p in db.Table1
                    join q in db.Table2 on p.table1ID equals q.table1ID
                   join r in db.Table3  on q.table2ID equals r.table2ID
                   group r by ?????? into g
                                 select new
                                 {
                                     total = g.Sum(p => p.value)
                                 };
                 
furthermore how can I make the +/- in the sum

right now I can do it in two ways

- by steps, but that is not an elegant way.
I am getting the query group by table3, and at the end I add all of them.

sum = query3.Sum(p=>p.HowMuch); but this one can have the +/-

- Also I can make the query directly.

but I don't like it. I would like to get a nice idea.

Thanks in advance

Diana