Unless your firm gives 100% offer rates consistently, you can't assume you'll get an offer. Odds are you will, but some firms notoriously hire far more people than they need and they choose the best out of them (or at least the ones that "fit in" best). You're not guaranteed an offer.So if you don't get an offer, and your grades go down, you're going to be in some trouble.
Also, have you thought about whether you might have any interest in clerking? If so, you need to keep your grades up this year.
Generally, once you have a job offer after the summer, most people relax and slack off more during 3L. There are too many unknowns still after 2L, and I think it would be foolish to get lazy now.