Will I lose weight if I walk 30 minutes a day at 3mph?

Yes unless... Yes, you will almost certainly lose weight over time unless you increase the amount of calories you are consuming. Unfortunately, many people reward themselves with a cookie or similar treat after exercising and wind up consuming more calories than they burn!
Exercise & weight. Very likely, yes. Effective weight loss is found when one combines a high protein, low calorie diet with good regular moderate exercise. Walking is ideal. The issue is that you must be walking fast enough to get your heart rate to about 120 BPM and keep it there for at least 20 to 30 minutes. This form of exercise then leads to burning off of fat and excess sugars.