Three days is the average time it takes to transition from glucose burning to ketosis. The reason is that it takes an average of 12 hours for the esophagus to process the last bit of food from your last meal. Its only when that happens do insulin levels begin to drop.
At this point the body turns to its stored carbohydrate (glycogen) in the liver and muscles for energy. It takes an average of 32 to 40 hours for the liver’s glycogen stores to be depleted whereupon the body goes after the glycogen in the muscles. Whereas its estimated that muscle glycogen stores are exhausted after 90 minutes of exercise. (I don’t know what the time is for non-exercise).
Since its easier for the body to breakdown glycogen for energy than fat, it will exhaust glycogen first before turning to fat.
This 3-day transition is difficult for a body not used to doing the transition. At first, the body will slow the metabolism down to adjust for the shutoff in calories. Typical symptoms are feeling cold, fatigue, headache, depression, etc. These symptoms can be exacerbated if the body has a refined carbohydrate/sugar addiction.
With fasting experience the body will shift from being “metabolically inflexible” to “metabolically flexible” and transition symptoms will become less severe or disappear altogether from succeeding fasts.
Just my sorta luck and skill! 
I hope you do well.
