Grate the ginger on a coarse grater; you can leave the skin on if you like. Put the ginger with its pulpy juice in to a bowl and sprinkle in your muscovado sugar.
Remove the rind from 2 of your lemons with a vegetable peeler, add to the bowl, and slightly bash and squash with something heavy like a rolling pin or a pestle. Just do this for 10 seconds, to mix up the flavours.
Squeeze the juice from all 3 lemons and add most of it to the bowl. Pour in the fizzy water.
Allow to sit for 10 minutes and then taste. You may feel that the lemons are slightly too sour, therefore add a little more sugar; if it's slightly too sweet, add a little more lemon juice.
Pass the ginger beer through a coarse sieve in to a large jug and add lots of ice and some sprigs of mint.