I am sure you can trade it in but it is an older car now so not all dealers will be interested. I suspect you will lose a lot on money if you do so as you will get bottom dollar for your car. If it is a good car I would keep it and get my money's worth out of it.
Unless you put a big down payment on it you will probably find that you don't end up with enough to pay off your loan.
Been there , done that and it is stupid to do so.
The dealer might add the deficit on to your new car loan which will make your payments really high and you will be even more upside down than before - viscous cycle!
Looking for a Used 3 Series in your area?
CarGurus has 29,781 nationwide 3 Series listings starting at $950.
Get More Money for Your CarBe seen by 20 million shoppers. Create Your Free Listing
Search BMW 3 Series Questions
BMW 3 Series Experts
Related Models For Sale
Used Cars For Sale