You have 3 liters of water heating up by 50 degrees celsius. It takes 4184 joules to heat 1 liter of liquid water by one degree Celsius, so it takes 627600 joules to heat 3 liters by 50 degrees. Dividing by 40000 joules per gram of fuel, it will take 15.69 grams of fuel per minute. Finally, for significant digits, we have to round to 16. grams of fuel per minute.

Edit: for most sciency uses, 1.6 times 10^1 grams of fuel per minute is likely the preferred way to write that.