I am glad that this topic has been resurrected. There is an ongoing debate by the line pilots at my carrier (at least out west here in PHX) about the purported saving in Config 3 landings. The company is saying that Config 3 is now the standard flap setting and full is to be only used when necessary for performance (wet, contaminated, Cat 3, etc.)
We have heard the same 20 kgs (50 lbs for us in the USA) savings mentioned but then someone brought up the point that all this is negated if you use greater than Idle reverse on landing.
In the Airbus (particularly the 321) you have far less stopping area on the disks (2 wheels per main gear) vs the larger Boeings. 737 and 319/320 are the same, of course. But pound for pound the Airbus empty weights are greater than the Boeing 737s. So you are using reverse thrust to compensate for the lesser stopping power of the Airbus brakes. The carbon brakes work best when heated but if you use Autobrake (recommended) then you have a 4 second delay before the brakes kick in. And you are coming in with cold brakes unless you are doing 45 minute sectors. Meanwhile the reversers are engaged and because we all want to stop before the end (always my main goal) we slide the revesers back to full. Look at the fuel flow - you will see about 1800 lbs/hr fuel flow per side while in reverse. Bring them to idle at 80 or 60 kts and you will have used your 50 lbs and maybe more. If you don't single engine taxi after your 3 minute cool down then you will burn 50 more. And don't forget, if you dropped the gear and went to final flaps at the FAF then you burned hundreds more pounds or kgs of fuel.
VFR I will drop the gear at 1500' agl and go to final flaps (3 or full) at 1000'. I will go managed speed at 5 miles out. I am spooled and stable between 1000' to 500' which is our VFR gate. Thats about as close to a constant idle descent as I can get and still be wiithin company parameters. Few F/Os I fly with will do that so I know that we are wasting far more fuel with early configurations and prolonged use of reverse than landing Config 3.