I had much more than 2-3kgs to lose after DD2 was born. Like Ilithyia I used calorie king and I allocated myself 2000calories knowing how active I could be. I also went back to the gym at 9 weeks. Just half hour sessions most morning, but a good trainer can give you a good idea of a suitable workout so that worked for me.
I was conscious of packing lots of nutrients into my 2000 calories as well and lots and lots of water.

After DS was born I tried a shake program that was reputably good with BF mums. He was 6 months old and a regular feeder. I followed their extra calories to the letter and while I did lose weight, I lost my supply so stopped straight away. I was so wary this time of that happening again. But it didn't.

So for me, real, wholesome, healthy foods and regular monitored exercise worked. Even starting with just regular walking did it for me with DD1, but as I age, the exercise bit seems just as important as the good food.