Would have done that but my balcony is at the back of my block of flats and my car is at the front. Only practical way is to let the hose run through the flat and out the bedroom window which would mean I'd need some pressure.
The 210 litre butt with a tap at the base will provide it's own pressure. I know because I used to have one on breeze blocks and fill my barrels underneath. Now I do the same with a 600L tank to my van tank via a 2.5" hose.
The flow from the standard tap on a butt is slow so go to a garden centre or hose supplier and get a bigger tap. Then attach enough hose (as big bore as you can manage bearing in mind you have to store it and run it through the flat) to it so that when you uncoil it through the flat and drop it "over the side" then there is enough to reach the tops of the barrels. Make sure you have another tap at the end of the hose.
This is how you operate it.
Make sure the tap at the end of the hose (bottom tap) is "shut". Uncoil the hose and drop it over the side out the back window. Open the tap in the barrel. Line up your empty barrels and fill each one by opening the bottom tap.
When you have enough shut the bottom tap, go upstairs and shut the barrel tap, go back down and open the bottom, go upstairs and pull in the hose and coil it round the water butt. Go to work.
Job's done and off you go! No need to siphon or pump. If you have an assistant to operate the upper tap it will be quicker.