Sum() In Empty Table Doesn't Return Zero

i have a query with this basic structure

SELECT id FROM table1,(SELECT SUM(field2) as total2 FROM table2) - (SELECT SUM(field3) as total3 FROM table3) as total

to understand better let's say that:
table1 are clients
table2 is money that enters for clients
table3 is money that exits from clients

i want to obtain a balance. the problem is that if table2 or table3 has no records for certain client, this client is exluded from results

any ideas?

i tried using ISNULL like this:

SELECT id FROM table1,ISNULL((SELECT SUM(field2) as total2 FROM table2),0) - ISNULL((SELECT SUM(field3) as total3 FROM table3),0) as total

but it didn't worked
this is the query
SELECT id, UPPER(apellido_titular) + ', ' + nombre_titular AS padre,
(SELECT SUM(facturas_items.importe) AS totalf
FROM facturas INNER JOIN
padres ON facturas.id_padre = padres.id LEFT OUTER JOIN
facturas_items ON facturas.id = facturas_items.id_documento
WHERE (padres.id = p.id) AND (facturas.fecha_vencimiento < GETDATE())) -
(SELECT SUM(recibos_items.importe) AS totalr
FROM padres INNER JOIN
recibos ON padres.id = recibos.id_padre LEFT OUTER JOIN
recibos_items ON recibos.id = recibos_items.id_recibo
WHERE (padres.id = p.id)) AS total
FROM padres p
WHERE (activo = 1) AND
((SELECT SUM(facturas_items.importe) AS totalf
FROM facturas INNER JOIN
padres ON facturas.id_padre = padres.id LEFT OUTER JOIN
facturas_items ON facturas.id = facturas_items.id_documento
WHERE (padres.id = p.id) AND (facturas.fecha_vencimiento < GETDATE())) -
(SELECT SUM(recibos_items.importe) AS totalr
FROM padres INNER JOIN
recibos ON padres.id = recibos.id_padre LEFT OUTER JOIN
recibos_items ON recibos.id = recibos_items.id_recibo
WHERE (padres.id = p.id)) > 0)
ORDER BY UPPER(apellido_titular) + ', ' + nombre_titular

sorry for my poor english