72 hours of fasting is more challenging than shorter fasts but also riskier.
For instance, if you aim to burn fat, you might burn more by fasting for 72 hours than by doing 48 hours, but you have a greater risk of dehydration, electrolyte imbalances, and effects on your metabolism.
Longer is not necessarily better when it comes to fasting, and you must listen to your body and consult a doctor before trying a long fast. Beginners should start with a short fast and see how it goes.