var works = 
           from a in A join b in B on a.aid equals b.aid into AB select new{a.enid, b.work}
           from ab in AB.DefaultIfEmpty() join c in C on c.enid equals ab.enid
           select new{workB = b.work, 
                      workC = c.work //?? 默认值 
           };

------解决方案--------------------

C# code

var query= from p in A
           join q in B
           on p.aid equals q.aid
           into temp
           from r in temp.DefaultIfEmpty()
           join s in C
           on q.enid equals s.enid
           into temp1
           from s in temp1.DefaultIfEmpty()
           select new 
           {
              q.work,
              r.work
           };