The third day of fasting is believed to be the time where the effects start to take place – that’s why some recommend fasting for 3-5 days for weight loss. It’s the time the body will be getting into the metabolic state of ketosis. Ketosis is the state where your body produces energy by burning stored fat.

By then, if you only drank water then you should lose around 8–12 pounds, but that weight loss can be temporary. The body will burn fat for energy more on day 4 or 5.

Your Insulin Sensitivity will improve so when you resume eating, your body will be more efficient at shuttling nutrients where they are needed (muscles and organ function) instead of storing them as fat.

Source

Here are other things to expect aside from losing weight from your body running on emergency supply:

You’ll be super hungry, then that’ll decrease and you’ll feel fine
Your breath may smell

Read more about them here.

I’m also attaching a link to a detailed blog I found about first-hand fasting experience.