In my own case if I binge drink for a few consecutive weekends its about 2 kilos of retention and I tend to lose it a little under 2 weeks time. So yeah 3 kilos sounds normal.
I was only drinking in the weekend and not during the week. So much effect?
Alcohol contains a massive amount of sugar. I'm not sure if you're aware, but sugar turns to fat when too much of it is presented to the liver (where it's broken down into energy). Therefore, the less sugar consumption, the more weight loss. And no, you don't need to see a doctor. The weight loss is a normal, and welcome, benefit of quitting drinking.
But losing 3 kilos in 3 weeks? is that not much?
Oh yeah I noticed something this too when I drink while dieting. A coworker brought it to my attention a while ago. When you drink it causes some water retention causing you to gain a little weight. The harder and longer you drink the more weight it is going to be. When you stop it tends to drop off in a few weeks time.
It could be a perfectly normal and healthy occurrence.