Wash the citrus well and pat dry — you're using the skin.
Zest the lemons, limes and oranges, taking only the colored layer, not the bitter white pith. Halve the fruit and set aside to juice.
Boil 250 g water with 250 g sugar until dissolved, then add 750 g cold water (or the same weight in ice) to make a warm 20% syrup.
Stir the zest into the warm — not hot — syrup and infuse 1 hour. Don't boil it; high heat drives off the citrus oils.
Strain, discard the zest, keep the syrup.
Finishing the lemonade
Juice the citrus and stir the juice into the syrup.
Add 1 liter of water.
Taste — make it brighter and sharper than you'd serve it, since the ice dilutes as it melts.
Chill well in the fridge.
To serve
Slice some citrus and pick a few sprigs of fresh mint.
Add to a jug with ice and stir.
Serve in chilled glasses with more citrus, mint and ice.
Notes
Stir in extra fruit pieces just before serving.
Top the base syrup with sparkling water for a citrus spritzer.
Add gin, vodka, or whisky for a cocktail.
Use the base syrup as a drizzle over cakes.
Freeze the finished lemonade in popsicle molds.
Swap the sugar for honey, weight for weight.
For a sugar-free version, use a sugar replacement — go by taste, not the measurements here, as replacements are much sweeter.
In sealed bottles, the lemonade keeps up to 5 days in the fridge. To store longer, freeze the syrup base (before the final water) for up to 3 months — thawed, finished lemonade separates, so freeze it only as popsicles.