Aplicado a tu select:
Código:
Select T1.codigo,T1.nombre,T1.sueldo,T1.codigocomentario, (Select SUM(T2.descuento) From Tabla2 As T2 Where T2.codigo = T1.codigo ) As descuento, T3.descripcion
From Tabla1 as T1 LEFT OUTER JOIN Tabla3 As T3 ON T1.codigocomentario = T3.codigo
WHERE T1.codigo= :pnum
Order By T1.codigo
ó
Código:
Select T.*,T3.descripcion FROM (
Select T1.codigo,T1.nombre,T1.sueldo,T1.codigocomentario, SUM(T2.descuento) As descuento
From Tabla1 as T1 LEFT OUTER JOIN Tabla2 As T2 ON T1.codigo = T2.codigo
WHERE T1.codigo= :pnum
Group by T1.codigo,T1.nombre,T1.sueldo,T1.codigocomentario
) As T LEFT OUTER JOIN Tabla3 As T3 ON T.codigocomentario = T3.codigo
Order By T.codigo
Saludos,
Jorge.