“Brunch replacing breakfast and dinner”

For a healthy metabolism, you should really aim to eat every 4 ish hours throughout the day.

Yes, this is probably fine, provided it is still a balanced meal and that you are not overeating to the point of discomfort. Conventional brunch meals tend to be high in carbs and fats and this may not be ideal on a daily basis (i.e., waffles, pancakes, bagels, etc.). Higher protein and veggies will provide longer satiation, so you might not be hungry until dinner time.

Absolutely, meal patterns vary depending on an individuals nutrition needs and personal schedule. Some people prefer smaller more frequent meals and others prefer fewer larger meals. As long as you are getting everything your body needs in a day then there is no wrong way to do it.