[jwmoreira]

Aplicado a tu select:

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

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.