Add the raspberries, blueberries, strawberries, sugar, water, lemon juice, and salt to a blender. Blend until completely smooth, about 45 seconds. For seedless sorbet, strain the mixture through a fine mesh sieve.
Pour the blended mixture into a clean Ninja Creami pint container, filling no higher than the max fill line. Secure the lid and freeze on a level surface for a full 24 hours, until solid.
Remove the lid and install the frozen pint in the outer bowl. Lock the bowl into the machine and press SORBET.
Check the texture. If it looks crumbly or powdery, add 1 tablespoon of water to the center of the pint and run RE-SPIN. Repeat once more if needed.
Scoop into bowls, garnish with fresh berries, and serve immediately.