3 kilos/6.6 lbs of elderberry is a bit light for 8 liters/2 gallons. I'd target 6 or 7 liters of initial volume. After you press the fruit the volume you get will be reduced when the fine fruit and yeast solids settle. Always plan for more volume than your secondary storage (glass jug or carboy the wine rests in while it clears) can hold. Put any extra in smaller bottles.
I'm assuming you're using 4 litter jugs for secondary. If you have access to smaller jugs (2 or 3 liter) you can adjust my comments to fit the space.
An alternative is to add a kilo of another fruit, and bump the volume up to 9 or 10 liters. Depending on the fruit, it may not change the taste much (elderberry is strongly flavored), and you'll have volume for two 4 liter jugs.
Yet another thought -- if you have access to grape concentrate, add a liter of concentrate + 2 liters water, bump the volume up to 9 or 10 liters. I did that with a gallon of elderberry concentrate -- the concentrate was supposed to make 5 gallons/19 liters, so I bumped the total volume up to 5.75 gallons (23 liters), which provided the volume to fill the carboy after racking off the sediment.
Regarding other fruits, for a wine that tastes like the fruit, plan for 2.25 kg (5 lbs) to 3.5 kg (8 lbs) of fruit per 4 liters water.