What happens if i do not have enough water intake for a few days? eg: i need 8 glasses per day, but i manage to have only 3 for a few days.

Nothing: Your a young guy and if you don't have any other medical conditions and don't take any diuretics you are unlikely to have any issues. And you don't need 8 glasses of water a day, that's an old wive's tale. The 8 glasses or 64 oz is an arbitrary number. You have thirst mechanisms that will make you drink when your thirsty to prevent dehydration. Like i stated, if young and healthy drinking 3 is ok.