In a small pot, bring raspberries, blueberries, strawberries, and sugar to a boil over medium-high. Remove from heat and mash with a potato masher. Pour through a fine-mesh sieve into a large container, pressing on solids to extract as much liquid as possible; discard solids. Refrigerate until cold, 30 minutes. Add seltzer and serve over ice with crushed raspberries and blueberries if desired.
This drink can be refrigerated for up to 3 days -- add garnish and ice before serving. Try using canning jars in place of pitchers or glasses for a casual party.