Can I defer my Visa interview by 3 months after completing my OFC (Biometric)

I have Fall 2018 admit from BU but got Spring 2019 admit from USC (my preferred Univ) after my OFC (Biometric) but before Visa interview. Since I-20 for USC will come only in September, can I defer my visa interview date to October? I understand I cannot modify my DS-160 and need to fill another one. What happens to my SEVIS, how do I transfer it? Can I do that in October?

Well, I am not sure about OFC part, you will need to check with Embassy or USTravelDocs for the same. If I were you, I would join BU and then transfer to USC for Spring, nothing is stopping you to do that. You can transfer some credits from BU to USC, if you do the right courses.