BMW 3 Series how much water should it take from empty please?

If system drained , you will need to put in correct amount of anti freeze and then top up as required . Then , when engine reaches working temp and circulated through heater , recheck and top up if needed .

check your hand book that should tell you HOW MUCH system holds.at a guess three litres of anti freeze should be about right

refer to vehicles hand book this will tell you the full amount of water and antifreeze content required to fill hope this helps