With a 3900 OBC rank in JEE Mains for CSE, you have a good chance to get into some NITs and IIITs, but probably not the top ones like NIT Trichy or IIIT Hyderabad. You can expect NITs like NIT Kurukshetra, NIT Jamshedpur, or NIT Raipur, and IIITs like IIIT Lucknow or IIIT Kota. Cutoffs change every year, so participate in counseling and choose wisely. Keep backup branches ready too for better chances.