Here is the Mapping file:

these 3 tables relationships as:
Customer->Address->contact
Customer.hbm.xml:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">
<class name="Customer.Customer, Customer" table="Customer">
<id name="Cust_ID" column="ID" type="Int32">
<generator class="identity"></generator>
</id>
<property name="Cust_UID" column="UID" type="Guid" />
<property name="Cust_Name" column="Name" type="String(50)" />
<property name="Cust_RegNo" column="RegNo" type="String(20)" />
<property name="Cust_Website" column="Website" type="String(67)" />
<set name="Cust_ContactPerson" inverse="true" table="Address" cascade="all-delete-orphan">
<key column="ID" />
<one-to-many class="Customer.Address, Customer" />
</set>
</class>
</hibernate-mapping>

Address.hbm.xml:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">
<class name="Customer.Address, Customer" table="Address">
<id name="Add_ID" column="ID" type="Int32">
<generator class="identity"></generator>
</id>
<property name="Add_UID" column="UID" type="Guid" />
<property name="Add_Name" column="Name" type="String(50)" />
<property name="Add_Language" column="Language" type="String(50)" />
<property name="Add_Postal" column="Postal" type="String(320)" />
<property name="Add_Address1" column="Address1" type="String(50)" />
<property name="Add_Address2" column="Address2" type="String(50)" />
<property name="Add_Address3" column="Address3" type="String(50)" />
<property name="Add_City" column="City" type="String(50)" />
<property name="Add_Postcode" column="Postcode" type="String(20)" />
<property name="Add_State" column="State" type="String(50)" />
<property name="Add_Country" column="Country" type="String(20)" />
<property name="Add_CustID" column="CustID" type="Int32" />
<set name="Add_Contacts" inverse="true" table="Contact" cascade="all-delete-orphan">
<key column="ID"/>
<one-to-many class="Customer.Contact, Customer" />
</set>
<many-to-one name="Add_Customers" column="CustID" insert="false" update="false" cascade="all" />
</class>
</hibernate-mapping>

Contact.hbm.xml:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">
<class name="Customer.Contact, Customer" table="Contact">
<id name="Con_ID" column="ID" type="Int32">
<generator class="identity" />
</id>
<property name="Con_UID" column="UID" type="Guid" />
<property name="Con_Type" column="Type" type="Int32" />
<property name="Con_Contents" column="Contents" type="String(255)" />
<property name="Con_AddressID" column="AddressID" type="Int32" />
<many-to-one name="Con_Addresss" column="AddressID" insert="false" update="false" cascade="all" />
</class>
</hibernate-mapping>

When i testing using this code:
Dim custs As Customer.Customer = session.Load(GetType(Customer.Customer), 2)

session.Delete(custs)

session.Flush()

session.Close()

The Error: "The DELETE statement conflicted with the REFERENCE constraint "FK_Contact_Address". The conflict occurred in database "

Pls help...thanks

You need to use the Cascade property to tell NHibernate that you want to delete the child relations when the parent object is deleted.

The other alternative is to set the table to cascade on delete within your DBMS.