Hi , i have 3 cdts namely A , B , C and three data store of the same i.e

See this documentation to see the one to many example:
forum.appian.com/.../Defining_a_Custom_Data_Type

Here is an example you can use:

<xsd:element maxOccurs="unbounded" name="testDataTypeC" type="test1234:test_data_type">
<xsd:annotation>
<xsd:appinfo source="appian.jpa">
                                                            @OneToMany(cascade=ALL, indexed=false)
                                                            @JoinColumn(name="data_type_b_id", nullable=false)
                                                  </xsd:appinfo>
</xsd:annotation>
</xsd:element>

In the above, since you aren't providing the definition of data type c in the same xsd, you will need to have this in your xsd. Please adjust the example accordingly.

<xsd:include schemaLocation="%7Bhttp%3A%2F%2Fwww.testing.com%2Fappian%test_data_type_c.xsd"/>

@michael : i have already made the xsd's and the table are mapped , But while writing to the table c , i am seeing that the reference of B is not taken . How do i solve this issue of making an entry to the data source ?

Himanshu, check that your variables are actually saved properly in to the cdt variable you are using in your write to datastore node. Also, what is your xsd definition of type B? This might show why when you write to C it doesn't have a reference to B.

@michael : I made a single XSD wherein i made three CDT'S A,B,C . In this A has a one to many reference of B , B has a one to many reference of C (like you've shown) .I was successfully able to populate the tables A and B using cdt of A and filling into data store A . Now i am trying to populate the table C so i am using the cdt of type B and feeding into data store C so it is showing an error and when i tried to populate C using cdt of type C and feeding into data store C ,it populates but the reference of B isnt populated .
I hope i am able to explain my doubt

Populating type B in to data store C won't work, as the type will need to match the data store entity.

Type C will only have references to B if it is written in to the database as a child of B. This is unless you control the foreign key relation in process by populating the foreign key yourself.