12 lbs in 3 weeks at a starting weight of 308 is largely water and glycogen (probably 7-8 lbs of it), with 4-5 lbs of actual tissue loss. This is completely expected and not concerning. As you noted, it will slow down — expect 4-8 lbs/month once the initial water loss phase passes.

The appetite suppression you're describing is typical of strong early responders. Enjoy it, but do make sure you're still getting adequate protein (minimum 100g/day) and staying hydrated. The risk at your intake drop is nutritional deficiency, not eating too little for weight loss purposes.