Hi Snehal,
You have to develop OData service in such a way that your header and item levels are associated.
Parent -> 1st entityset
Child 1 (2nd entityset)
Child 2
Child 3
Once you have the service ready for (Expanded entityset using $expand). You can then use it to consume in your UI5 code. The best example for this usecase will be approve purchase order, where you have multiple items assigned to a purchase order. Check out Fiori demo edition on how to. Refer standard code and replicate the same usecase.
Few blogs :
Let's code association/navigation and data provider expand in OData service!
http://www.saplearners.com/association-and-navigation-in-sap-netweaver-gateway/
Regards,
Tejas