Triplets and Trios (and the number three, in general) are a very popular motif in the history of world religions. Three is a magic number of course, and these ladies and gents are known for sticking together like glue. Some of them are even considered a single entity (think of them like parts of an egg: yolk, white, and shell—unique but inseparable). Their status is pretty elevated above the common playground dwellers, and they are treated with a combination of reverence and fear. The Morrígan and her sisters, Badb and Macha, make up the triple threat known as the Morrígu.
Commonly known as the Fates, these three sisters dictate, well, your fate. They can make your life long and prosperous or cut it tragically short. Clotho, Lachesis, and Atropos are often described as women in three different stages of life (young, middle aged, and old, respectively). We recommend getting on their good side.
In the Christian tradition, the Father, Son, and Holy Spirit/Ghost are viewed as three parts of a whole (i.e., God). St. Patrick compared their triple composition to a three-leaf clover, with each of the leaves representing one aspect. Depending on who you ask, this trio is either merciful or vengeful, but the consensus is this: superpowerful.
This Greek goddess hangs with ghosts and is loved by those who dig magic. Hecate may not be three separate people, but she is generally depicted as a goddess with three heads/faces. We think that counts.
There are a lot of norns wandering around, but there are three norns that are more important and well-known than the others. Urd, Verdandi, and Skuld hold posts very similar to the Greek Fates. These ladies weave the fate of Norsemen and women while tending to the health of the World Tree.
In the Hindu tradition there are two super-sets of three. The gents, Brahmā, Vishnu, and Shiva, make up the Trimurti, and the ladies, Saraswati, Lakshmi, and Parvati make up the Tridevi. The Trimurti, in particular, reigns supreme in Hindu. They are responsible for the creation, life, and destruction of everything. No big deal.
Medusa is the most famous (infamous?) of these three gnarly sisters. Euryale and Stheno are the other two. All three have snakes for hair and are so ugly, they might turn you to stone—or that's what they say. They're not very popular, so who knows? Could all be gossip. Either way, someone should probably call One Direction to show these girls some love.