How do I calculate the distance a car can travel based on a fixed fuel consumption rate (in kilometres per litre), the amount of fuel in the tank and the distance already travelled?

Assume that the tank is empty and we add some fuel at the start.

For example, a car has a fixed fuel consumption rate of 6 k/l and I add 3 litres of fuel and I've already travelled I don't know, lets say, 5km.
How do I calculate the quantity of fuel left in the car then after adding the fuel and then travelling over x amount of distance.

Your question is a little strange. You say that the tank was empty but you travelled 5km. Did you push the car (just kidding)?

If you put 3 litres of fuel in the car and you get 6km/litre, then you should be able to go 18km with this fuel. If you have travelled 5km out of this 3 litre fill up, you can still travel another 13km.

Alternatively, you could say that you would use 0.1666 litres per km. If you have travelled 5 km, you would use 0.83 litres, leaving 2.1666 litres left in your tank.