How should I break the fast? What should I eat after 3 days fasting?

Knowing how to break the fast is more important than the fast. 

Any fast should be broken very very gradually. To understand why, let's take an example. When you start a car, you first have to put it on. the 1st gear, then 2nd gear, then 3rd gear and then 4th gear. If you put the car suddenly on the 4th gear, it will a shock and engine will fail. Same way, when you start eating after a fast, you first have to put your body on the 1st gear, then 2nd, then 3rd and finally 4th.

The first gear = light, water-rich fruits
The second gear = vegetables/salads
The third gear = grains (roti, rice, pulses, legumes, etc)

Hence, after a fast you should consume light, water-rich fruits, followed by salads and then finally, grains.

Depending on how long your fast was, it should take you a few days to break the fast. Time taken to break the fast should be of half the duration of the fast. 

A 7 day fast should take at least 3.5 days to break the fast
A 3 day fast should take at least 1 1/2 days to break the fast

So, if you are fasting for 3 days, your diet on the 4th day should as shown in the chart below - 

AFTER A FAST, YOU MUST NOT HAVE A HEAVY MEAL AT ONCE
If, after a fast, you eat a heavy meal at once, you will lose all the benefit gained during the fast. In fact, eating a heavy meal after a fast can be dangerous. 

After a fast, the digestive fire becomes very weak. if we suddenly put a lot of food over it, it will completely extinguish and cause a lot of trouble. We should slowly and slowly increase the digestive fire by very gradually giving it food.