What grades do I have to get in order to avoid getting my acceptance rescinded?

All B grades would be a 3.0, not 3.7. The drop in question refers to previously unreported grades to the college, not the effect on your overall GPA over 8 semesters.

3.8 → 3.0 would not be risk-free. Keep doing as well as you can so that even if you drop a little, it would not be all that risky.