Is there any word to designate a meal that serves as breakfast, lunch and dinner? (i.e. only meal of the day)
Typically the meal is named for the time-frame in which it occurs.
- Breakfast: early morning
- Lunch: midday
- Dinner/Supper: early evening (although dinner can also be noon to early-afternoon especially on Sundays and holidays, and whether the evening meal is called dinner or supper varies regionally as well.)
If the meal falls outside the "normal" time-frame it is modified: late dinner, late breakfast, early lunch, late lunch.
Terms like Brunch combines two elements of a meal- time-frame and menu. Brunch, combining Breakfast and Lunch, is usually eaten mid-morning, in-between the normal breakfast and lunch time-frame, and usually consists of both breakfast and lunch foods.
There's not as much of a distinction between Lunch and Dinner menu items so there really isn't a separate word combining Lunch and Dinner. It's either a late lunch or an early dinner (depending which meals you've already eaten or plan to eat.)
To combine all three meals into one you'd typically modify it with "Big" and then choose the term appropriate for the time-frame in which it was taken.
A "Big Breakfast," a "Big Lunch", a "Big Dinner" etc.
There's no need for a separate word because all the time-frames are adequately covered. For example:
We'll be eating a big lunch tomorrow so I'm going to skip breakfast and I probably won't be hungry for supper later.
I ate such a big breakfast that I think I probably won't eat again until tomorrow.
The brunch at Hotel del Coronado was huge! I couldn't possibly eat anything more for the rest of the day.
Dinner is at 1:00pm tomorrow. We're going all out, so come hungry and be prepared to not eat for the rest of the day.