"three degrees hotter" is an interval of three degrees, and not an absolute value along a scale. So you'd take your original temperature and add an interval of

"three degrees hotter" is an interval of three degrees, and not an absolute value along a scale. So you'd take your original temperature and add an interval of 3 degrees to it. 0 degrees Celsius has a heat value (273.15 K). If you double it (i.e. add 0 degrees Celsius to it), you get 546.3 K -- which is 273.15 degrees Celsius. What it really comes down to is that if you're going to perform math on temperatures for scientific reasons, use Kelvin. And if you're adjusting a temperature by an interval, leave out the units from the math completely.