If you like, you can see that there is only one right way to do it, and of course it is the way we do it out here
If something is a 'few degrees west of north', then its bearing is something like 357 degrees. So in the UK, the true bearing of magnetic north is 357. In other words when your compass says 0, you know the true bearing is 357, and in general you need to add 357 (modulo 360) to your compass bearing in order to get the true bearing. You just chose to think about it as subtracting 3.
Whenever I have to deal with declination I think about what happens at zero degrees; it will be the same for any other bearing. I inevitably confuse myself.