It'll boil, I can maintain a weak boil on a 5gal batch with my single 2000w heatstick. For 3000W I'd guess around an hour to reach a boil. You can use this
calculator to figure out how long it'll take to boil based on your water temp.
As for how much you can boil, I don't know for sure but at some point you'll just get tired of waiting for the boil to happen.