Don't cut out carbohydrates. It's the bodies main supply of energy, and you need energy to produce milk. If you are that worried about weight gain, then work out how many calories you need to be consuming a day as a breastfeeding woman, then simply try to fit your cravings into your calorie allowance, and in a sensible way.
For example, if you are craving sweet stuff, then try having a cup of low calorie hot chocolate instead of a chocolate bar. It really helps with the craving, and is a fraction of the calories. Or get some very dark chocolate (70% cocoa solids), which has much less sugar in it, so you only need a square and you're done as it's so rich.
Try having porridge made with milk or water in the morning, and have some raisins and honey in with it.
For vegetables, it doesn't get much easier than just cutting up a bunch of veg and shoving it in the oven with olive oil and rosemary. You can have it as a side serving to pretty much anything!
You are only three weeks post partum. Your body needs energy, which might explain your cravings. If you are genuinely THAT worried about your figure after not putting on excess weight in pregnancy, and presumably starting at a healthy weight, then I genuinely suggest you speak to your doctor about it, as it's not healthy to be that obsessive at the expense of your health and recovery.