If i'm eating three slices of pizza, is it better to eat them all at once or spread them throughout the day?

It's best: To eat them when you are truly hungry and stop eating them when you are not. Otherwise, it doesn't matter.

It depends: Unless you're diabetic and therefore at risk for an uncontrolled spike in blood sugar, the only difference eating 3 slices of pizza at once will make is how your stomach feels afterward. The caloric effect is the same no matter how you divide them up.