The Charge 3's module weighs in at 20 percent lighter than that of the Charge 2, so it won't weigh down your wrist when you're wearing it. I barely noticed it on my wrist when I paired it with Fitbit's woven bands, which are made of a feather-light fabric material. If you're looking for the band that both complements the device's weightlessness and remains fashionably flexible enough to go from the gym to the office, I'd go with a woven option instead of the standard silicone one.
Yes, it can monitor your health and fitness. You should choose one that has a pedometer to count your steps. An accelerometer is helpful in determining acceleration. Acceleration can be used to detect when you fall. A heart rate sensor can be used to monitor your heart beats. Some will report the amount of calories you consume and even display a report of your health at day’s end.