شما یک کوئری شبیه به این نیاز دارید :

var query = (from p in kharid
join n in goruhkharid on p.id_group_factorkharid equals n.id_tarafhesab into pn
where pn.id_group_factorkharid == 12
from c in pn
join d in tarafhesab on c.id_tarafhesab equals d.id
select new {d.name_tarafhesab}).Single();