Hello Bhargavi,

In Karnataka 3B Category is considered under OBC category. In JIPMER PG application form, In the Category field, you will have to select Other Backward Class (OBC- Non Creamy layer) and then you will have to select the state you belong to (Karnataka).
In the next field, it will ask for your Entry No.
So, you can refer to the link below to check the Central List of OBC details and fill the Caste/Community to which you belong.

https://drive.google.com/file/d/1McGRfNlPdYbRldR5EHFLdya3UBs6UPYL/view

Hope this helps. Do let me know if you have any other query.
Good Luck!