There is no graph or equation for me to solve :(
The temperature that 12%C is hotter than 16%C is 4%C
This is possible for a smaller watering can. 3 liters is about 3/4 of a gallon (imagine a jug of milk).
It is equivalent to 3000 ml, which would also be 100 ounces of water.(5 -20 oz bottles of water).