Wash limes under cool running water and zest two limes using a microplane, avoiding the white pith.

Cut all limes in half and juice them to yield 200 ml fresh juice; strain through a fine mesh strainer to remove pulp and seeds.

Combine 200 ml water and 200 g sugar in a saucepan over medium heat.

Stir constantly for 5 to 7 minutes until sugar fully dissolves and syrup is clear.

Remove saucepan from heat and let syrup cool to room temperature, about 10 minutes.

Stir in strained lime juice, grated lime zest, and citric acid (if using) until well combined.

Pour mixture through a fine mesh strainer into a sterilized 500 ml glass bottle to remove zest solids.

Seal bottle tightly and chill in refrigerator for at least 2 hours before serving.