Place the berries in a colander. Rinse under cool running water, removing any damaged berries and stems.
Fill a pot with 2 inches of water. Bring the water to a boil.
Place the elderberries inside a steamer basket. Place the basket in the pot so the berries are above the boiling water.
Place a lid on the steam pot. Steam the elderberries for one minute to soften the skins. Plunge the berries into a bowl of ice water after steaming to stop the cooking process by cooling the berries rapidly.
Dissolve 3 cups sugar in 4 cups water inside a saucepan. Bring the mixture to a boil, stirring constantly. Remove from the heat and allow the mixture to cool.
Drain the ice water from the berries. Place the fruits inside a freezer safe container, filling the container to within an inch of the rim.
Pour the sugar mixture over the top of the berries until they are just covered. Place the lid on the container.
Store the elderberries in the freezer for up to one year. Thaw in the refrigerator for 24 hours before using.