在SSRS中使用3个数据集的Lookupset函数

Question

如何在具有3个数据集的SSRS中使用Lookupset功能

=LookupSet(Fields!Account_Number.Value,Fields!Account_Number.Value,
  Fields!Referral_Date_Created,"ReferralsDS")
  (LookupSet(Fields!Account_Number.Value,Fields!Account_Number.Value,
     Fields!Referral_Date_Created,"ReferralsDS").Length -1)