I agree, if you're only running 3 miles I would think that you really don't need one, but that's a personal hydration thing. But all-in-all, I use one when running anything longer than 10 miles, and I find it to be quite useful and not bothersome at all. I don't like wearing a water belt and having something bouncing around my waist where I feel as if the weight is too low. Wearing a CamelBak allows me to carry the weight closer to my body because it sits on my back and the straps around the front of my chest help hold it there. It took me a bit to get used to, in which case I did wear one for my short 3-5 mile runs. Most of that getting used to phase was getting used to the weight, strap tightness and learning how to drink out of it. But once I did, I have been very grateful for having one during my long runs.
Hope this helps, let me know if you have any more questions.