A Boy a Girl and a Baby Family - Television Tropes & Idioms

There's also the family pets, but who's counting?

Or a Boy, a Girl and Baby siblings. Not only are families with three kids common in TV land, but often they are made with a boy, a girl and a baby of either sex (nearly 50/50 ratio).

And that kind of family has immutable rules:

The two elder siblings are very close in age, between one and three years. The eldest sibling can be either sex, but it is usually a girl. The odds of her being a girl seem to go up with the size of the age gap between the two older siblings.
On the opposite end of the scale, the youngest is far from them, between five and 14 years younger.
The boy and girl are between ages 7 and 15 years: not too young children and not too old teens, either.
The baby is not an infant: more like a toddler between 6 months and 2 years old, but before the age of learning to talk.
Just because one can't walk or talk yet does not mean that the infant sibling is good for nothing or powerless. On the contrary, the infant is often shown to be the most powerful in the family (if applicable), or a Brainy Baby, or a Little Miss Badass. Even when they are only the Plucky Comic Relief for most of the story, the baby can save the day at the very end as the Big Damn Hero.

The reason? Brother and Sister have the target audience's age and allow everyone in the audience to identify with the sibling of the same gender. The baby could also be a substitute for a child while the siblings are "parents". It also allows the writers to have starring characters for Always Male and Always Female plots or tropes, and the baby as a child substitute, a third man or younger sibling role. The sex of the baby varies as gender doesn't make as much of a difference for infant characters. Alternatively, the baby can take the role of a Team Pet.

Examples

Manga

In the manga Kocchi Muite Miiko, the Yamada family has Miiko, Mamoru, and their baby sister Momo.

Fanfiction

At the very end of the Superjail! fanfic Extended Stay, the Warden and the Mistress's family has been established as this with their mixed-gender twins Matilda and Ethan and the "baby" Edward. (Although at this point, the twins are 20 years old; their brother being five years younger than them.)
- Foreshadowed earlier in a dream Mistress has in Chapter 11.

Film — Animated

Violet, Dash and Jack-Jack Parr in The Incredibles.
Fievel, Tanya and Yasha Mouskewitz in An American Tail.

Film — Live-Action

Pugsley, Wednesday and Pubert Addams in the second movie of The Addams Family.
Nick, Amy and Adam Szalinski in Honey, I Shrunk The Kids franchise (although they didn't appear together).

Literature

The Baudelaire siblings from A Series of Unfortunate Events.
The 10-year old twins Kestrel and Bowman Hath and their infant sister Pinto in The Wind Singer. In the later books of The Wind On Fire, the characters age.
The Balicki family from The Silver Sword.
The Berenstain Bears, at least since the addition of baby Honey Bear.
Tash and Zak Arranda in Galaxy of Fear are being looked after, with some difficulty, by their uncle. When they find a baby they want to adopt him and make this, but he's actually a Tykebomb.

Live-Action TV

Robbie, Charlene and Baby Sinclair in Dinosaurs.
Good Luck Charlie: With a fifth child on the way, the family will have two boys, two girls and a baby.

Newspaper Comics

Zoe, Hammie and Wren from Baby Blues.
For Better or for Worse, back when April was still a baby.
For a short time after Rerun was born, there's Lucy, Linus and Rerun van Pelt in Peanuts.
The family from FoxTrot would have followed this back when Jason was a baby, although we never get to see that time except in the rare Flash Back.

Webcomics

Holy, Polo and Elis in Irregular Elis.
Riyuokoan, Mimi and Enigma in Fiyora Nya.
Rudy, Lindesfarne and Coney Dewclaw of Kevin & Kell.

Western Animation

Bart, Lisa and Maggie Simpson in The Simpsons.
Chris, Meg and Stewie Griffin in Family Guy.
- Cleveland Jr., Roberta and Rollo in The Cleveland Show.
Arthur, DW and Baby Kate in Arthur. A bit of a difference from the norm in that Baby Kate is still just a baby and doesn't really do anything special, at least until the show introduces the idea of babies and animals all talking to each other and being able to understand only each other. A lot of the other time, it's easy to forget that that there even is a baby in the house and sometimes it seems like the writers do just that, especially in some of the earlier installments.
Doug, Judy and Cleopatra "Dirtbike" Funnie in Doug when the last one was born in the Disney version.
PB&J Otter: Peanut (brother), Butter (baby sister) and Jelly Otter (sister). Another slight aversion from the normal rules - Baby Butter is old enough to talk a little at least, and while the ages aren't certain, she may be only about three or four years younger that Peanut and Jelly.
The Lionhearts also plays this straight.
Birdz too.
Buzzbee and his familly on The Hive
Subverted in The Buzz On Maggie with two boys, a girl, and a baby.
Animaniacs has this with Yakko, Wakko, and Dot, slightly altered with all three being able to talk, and the two boys (Yakko and Wakko) are both older than their sister Dot.
The Legend of Korra, Sequel Series to Avatar The Last Airbender, reveals that Aang and Katara had three children together: Kya, Bumi, and the "baby" Tenzin.