In a large saucepan, toss strawberries with sugar. Let sit, stirring occasionally, until sugar is dissolved, 1 hour. Add lemon juice and bring to a boil. Cook, stirring, until berries are soft, 8 to 10 minutes. Add raspberries and blueberries and cook over medium, 25 minutes. Transfer to a clean glass jar, secure lid, and let cool to room temperature.
Refrigerate, up to 3 weeks, or freeze, up to 6 months.