I descaled my duetto recently - more out of curiosity than necessity. I use filtered water, so there's barely any scale build up.
I had to drain it about 5~6 times to get rid of the taint taste... it was a pain in the butt.
The steps were quite similar to yours, except I made sure the steam boiler was filled with water. The steam boiler is a little bit trickier (a wrote some instructions for it -
http://coffeereview.tumblr.com/post/111...structions)
I have used citric acid with my old machine. Have you ever seen citric acid perform in a kettle? it works quite well - scale is instantly dissolved and nothing is dislodged. However, given that the duetto is a little bit more expensive than a kettle, I didnt feel like taking a risk. The same goes with your GS3... I would recommend you buy an espresso machine descaler just to be safe. Just make sure you buy enough for the steam boiler and brew boiler. The descaler will bit slightly more expensive, but hey... you paid much more for your machine and this is something you do about once a year.
Good luck!