Blend strawberries, sugar, lemon juice, and water until smooth. Strain through fine mesh strainer. Press pulp to extract all liquid. Discard pulp. Refrigerate.
Combine lemon juice and sugar in a jug. Stir until sugar fully dissolves. Add cold water and pinch of salt. Stir again. Taste and adjust. Refrigerate.
Blend blueberries, sugar, lemon juice, and water until smooth. Strain through fine mesh strainer. Press pulp to extract all liquid. Discard pulp. Refrigerate.
Once all layers are fully chilled, fill each tall glass three-quarters full with ice cubes.
Pour strawberry layer slowly into each glass over the ice, filling one third of the way up.
Place the back of a long-handled spoon just above the strawberry layer surface. Pour lemonade slowly over the back of the spoon until two thirds full.
Place the back of the spoon just above the lemonade surface. Pour blueberry layer slowly over the back of the spoon until the glass is full.
Do not stir or shake. Handle assembled glasses gently.
Garnish with strawberry slice on rim, blueberries on top, lemon wheel, and mint sprig.
Serve immediately with a straw and instructions to stir before drinking.