I'm on a diet is it okay to eat more calories at breakfast than lunch and dinner ?

Makes sense to so that because the calories in the morning will be burnt over a longer period.
As long as the calorie intake does not change at the end of the day

Absolutely. Yes, it is always important to have a big breakfast. So go ahead and eat most of your calories then.

Sure. Go for it, as long as you stay within your calorie intake at the end of the day.