Hi Chitra,
I strongly recommend using 7.0 data staging technology in all cases from now onwards. It's superior and has a lot of advantages compared to the obsolete 3.x technology. At some point in the future you can expect that SAP won't support 3.x anymore.
If you have existing 3.x data flow, you migrate those data flows one by one. You don't have to migrate them all at the same time.
If you have a data flow which starts with a DataSource in an SAP source system, then you will have to migrate the DataSource anyway. InfoPackage will load into PSA. From here you can migrate the complete 3.x data flow or create the new 7.0 objects from scratch. Whatever is most convenient to do.
Best regards,
Sander