问如何在LINQ C#中联接三个表时执行分组依据

EN

var result = (from cm in DB.BRAND_NAME_MAP_COMMENTs
          join R in DB.BRAND_NAME_MAP_MASTERs on cm.BRAND_NAME_MAP_ID equals R.BRAND_NAME_MAP_ID
          join K in DB.V_BRANDNAME_INGREDIENT_MAPs on R.BRAND_NAME_MAP_ID equals K.BRAND_NAME_MAP_ID
          orderby cm.UPDATED_TS descending
          group K.PFC_DESC by K.RAND_NAME_MAP_ID into L  //this where the problem is                          
          select new WorkflowComment
          {
              NAMEVersion   = L.PFC_DESC + "-" + K.PFC_VERSION,
              BRAND_NAME    = R.BRAND_NAME,
              HC_BRAND_NAME = R.HC_BRAND_NAME,
              NAME          = K.PFC_DESC,
              //VERSION     = K.PFC_VERSION,
              COMMENTS      = cm.COMMENTS.Replace("\n", "<br />"),
              UPDATED_BY    = cm.UPDATED_BY,
              UPDATED_TS    = cm.UPDATED_TS
          }).ToList();