Late ultrasound measurements are notoriously off with weight estimates, and you have a while to go yet. You could easily have an average sized baby (I'd call average healthy sizes 6-10 lbs). A baby that ends up very very large (over 11 lbs) can present some extra challenges, making labor take longer or the pushing stage more difficult if the baby's positioning isn't ideal also (check out spinningbabies.com about that). A drug free labor and freedom to get in upright positions makes it easier no matter the size. If you don't want a c section, be firm about doing a trial of labor even if baby is large and try to minimize interventions that might lead to one (pitocin, pain meds, doctor breaking the water), and consider hiring a doula to help you in labor. Also watch out for any signs of gestational diabetes, don't overeat sugars or refined carbs whether you have that or not. Don't restrict your eating though, get lotsa protein and healthy foods as you like.
Your baby isn't actually 3 lbs. 11 oz. That is just what your baby is estimated at - and those estimates can be way off. My babies were both preemies and I had ultrasounds within 2 days of their births. Both were born about 20% less than what they had been estimated to weigh.
Don't worry about it, really. It's not an exact science, and it's not something that you need to do anything about.
It's normal for 3rd trimester ultrasounds to be off by 2 lbs or more. Really, they're just guesstimates (heavy on the GUESS). Two friends of mine were repeatedly told they were having big babies, and one baby weighed just under 8 lbs at birth, and the other just over 8 lbs. You don't need to do anything, except not worry about it!