Question about three Primary Keys on a DAC - acumatica

Question

In my case I have three Primary Keys,
Primary Key 2 is based on Primary Key 1,
Primary Key 3 is based on Primary Keys 1 and 2
My Question is how do you unset value of a Primary Key DAC Field?
Im trying to work on if I select on the Primary Key 1, Primary Keys 2 and 3 will be unset. I tried cache.SetValueExt<DAC.Field>(data,null); still no luck.
#region FundType
[PXDBString(3, IsKey = true, IsFixed = true, IsUnicode = true, InputMask = "")]
[RMPCFCustodianSetup.FundTypeList]
[PXDefault(RMPCFCustodianSetup.PCFDocType.PCF)]
[PXUIField(DisplayName = "Fund Type", Required =true)]
public virtual string FundType { get; set; }
public abstract class fundType : IBqlField { }
#endregion
#region ReqPayClassID
[PXDBString(10, IsKey = true, IsUnicode = true, InputMask = "")]
[PXSelector(typeof(Search<RMPCFSetup.classID,
Where<RMPCFSetup.fundType, Equal<RMPCFRegister.fundType>>>),
typeof(RMPCFRegister.reqPayClassID),
typeof(RMPCFRegister.descr),
DescriptionField = typeof(RMPCFSetup.descr),
Filterable = true
)]
[PXForeignReference(typeof(Field<reqPayClassID>.IsRelatedTo<RMPCFSetup.classID>))]
[PXUIField(DisplayName = "Request Class", Required =true)]
public virtual string ReqPayClassID { get; set; }
public abstract class reqPayClassID : IBqlField { }
#endregion
#region RefNbr
[PXDBString(15, IsKey = true, IsUnicode = true, InputMask = "")]
[PXSelector(typeof(Search<RMPCFRegister.refNbr,
Where<RMPCFRegister.fundType,Equal<Optional2<RMPCFRegister.fundType>>,
And<RMPCFRegister.reqPayClassID,Equal<Optional2<RMPCFRegister.reqPayClassID>>>>>),
typeof(RMPCFRegister.fundType),
typeof(RMPCFRegister.reqPayClassID),
typeof(RMPCFRegister.refNbr),
typeof(RMPCFRegister.extRefNbr),
typeof(RMPCFRegister.descr),
typeof(RMPCFRegister.docAmt),
typeof(RMPCFRegister.dateRequested),
typeof(RMPCFRegister.docStatus)
)]
[PXUIField(DisplayName = "Ref Nbr",Required =true)]
[AutoNumber(typeof(Search<RMPCFSetup.classNumberingID,
Where<RMPCFSetup.fundType, Equal<Optional2<RMPCFRegister.fundType>>,
And<RMPCFSetup.classID,Equal<Optional2<RMPCFRegister.reqPayClassID>>>>>),
typeof(AccessInfo.businessDate))]
public virtual string RefNbr { get; set; }
public abstract class refNbr : IBqlField { }
#endregion`