Combine blueberries, sugar, and lemon juice in a small saucepan. Cook over medium heat until berries break down, 5–7 minutes. Cool.

Toss strawberries and raspberries with a teaspoon of sugar if they’re tart.

Layer pound cake cubes, whipped topping, mixed berries, and a spoonful of blueberry sauce in clear cups. Repeat to fill.

Chill at least 1 hour so cake soaks flavors. Top with extra sauce and mint before serving.