var query= from us in tbOne
           join ca in tbTwo
           on us.tbOneId equals ca.tbOneId
           into temp
           from ca in temp.DefaultIfEmpty()
           join u in tbThree 
           on ca.tbThreeId equals u.tbThreeId
           into temp1
           from u in temp1.DefaultIfEmpty()
           select new 
           {
              us.tbOneName,
              ca.tbTwoName,
              u.tbThreeName
           };