Breakfast/lunch/dinner are how I describe three, not always square meals.
I eat breakfast late, so sometimes that runs into lunch for me, I haven't eaten it yet.
Two exclusions apply, when dinner was and is in the middle of the day at school, forever known as "school dinners" Christmas dinner - in our household around 3 pm.
Lunch can be something light, sandwich, soup, jacket potato, or, if out to lunch the most substantial meal of the day.
Tea is an afternoon affair, pot of tea, scones cakes etc.
Dinner in the evening, main meal of the day, if something more substantial hasn't been had at lunch time.
Some people refer to the evening meal as supper. Although, as others have stated, that could also be some morsel type of meal in the evening which tends to conjure up a Dickensian vision. for me, I don't know why.
I perceive many of the differences are regional.