Often, there are numbers which are used over and over again by a society, and as a result, people attach supernatural significance to them. This causes people to use them even more frequently, and thus you have a feedback mechanism. This is numerology. For instance, in Mesopotamia, the number 40 was viewed as synonymous with "many". Thus you have the following examples.
In the Babylonian creation myth, Marduk defeats Tiamet and her 39 monsters. Counting Tiamet herself, that's 40.
Noah's Ark sailed around for 40 days.
Jesus wandered through the wilderness for 40 days.
After Jesus returned to Earth after visiting Hell, and then left Earth a second time, it took him 40 days to reach Heaven.
In "A Thousand and One Nights", you have Alibaba and the 40 thieves.
The Coptic Church in Egypt celebrates Advent for 40 days.
The Muslim holiday of Ramadan lasts for approximately 40 days, although not exactly 40 days.
Three symbolizes male, and four symbolizes female. 3 x 4 = 7. Seven is a perfect example of one of these magical numbers. I couldn't make a list of the times that seven is used. You have seven days of the week. If you break a mirror, you have seven days of bad luck. In Victorian England, when making plum pudding, each family member would stir the bowl seven times. In witchcraft and demonology, they use the word seven as frequently as possible. You have a run-away chain reaction where the more people use it, the more significance is attached to it, which causes it to be used more frequently, which makes it seem even more magical, etc. 3 x 4 = 12. Thus 12 was a magical number. You have the twelve signs of the Zodiac. For this reason, Jesus was given twelve apostles. Later people noticed that there were 13 people present at the Last Supper. Thus 13 was viewed as an unlucky number. Since the Last Supper was on Good Friday, Friday the 13th was an unlucky day. Thirteen witches make a coven. Do you see how the significance attached to 13 arose accidentally due to the significance previously attached to 12? However, you can choose any number, and there are things of that number, and it's random which ones people happen to notice. Here are a few examples.
1. - One god of Judaism
2. - Yin and Yang of Taoism
3. - The Trinity of Christianity, the three wise men
4. - The four horsemen of the Apocalypse
5. - Five points on the pentagram of demonology
6. - The number of the Beast, 666
7. - Many examples, the seven seas
8. - The Noble Eightfold Path of Buddhism
9. - Cats have nine lives
10. - The ten commandments
If you want a small number that significance is attached to, the traditional choice is three. If you have two things, it implies two opposing forces in balance, such as good vs. evil, male vs. female, day vs. night, etc. Even though that is often used, frequently, you don't want to convey that concept. If there are four things, that's to many. You are less likely to know them all completely or visualize them all at once. Thus three is a common number. A perfect example is the Trinity of Christianity: the Father, the Son, and the Holy Ghost. What is the Holy Ghost? It never appears in any other context. The Holy Ghost was invented purely to make the number three. If all you had was the Father and the Son, it would imply that they were two opposing forces. Satan is often portrayed as having three faces. In Christianity, you have the Trinity (Father, Son, Holy Ghost), the three sons of Adam and Eve (Cain, Abel, Seth), and the Three Wise Men (Melchior, Gaspar, Balthazar). In children's fairy tales, you have Goldilocks and the Three Bears, the Three Little Pigs, and the Three Billy Goats Gruff.
Characters with some reference to the number three are extremely common in Greek Mythology. Cerberus has three heads. Part of the pediment from a temple on Acropolis in Athens, built in 570 B. C., and then destroyed by the Persians in 480 B. C., shows Hercules and a weird three-bodied monster. The monster has three male torsos, meaning three men from the waist up, all of whom share one lower body, which as the form of twisted snakes. We don't even know what this monster was called. You see the element of three. Many people or creatures in Greek Mythology take the form of three separate, and often identical, individuals. The main gods take the form of one person. You don't have three Zeuses or three Apollos for instance. Human characters, such as Hercules, appear as one person. Completely nonhominoid monsters, such as Scylla or the Chimera, appear as one. However, very minor gods and goddesses, or supernatural creatures that are roughly hominoid, very frequently appear in threes.
There are some exceptions. Oedipus defeats one Sphinx, and Odysseus defeats one Cyclops, but the number of threes is amazing. The triplifying is completely redundant. The story of Perseus would be identical if there were one Gorgon and one Graeae. The story of Orestes would be identical if there was one Fury.
Here I shall give examples of some of the characters that appear in Greek Mythology in threes. The first, the Muses, is not actually three but nine, but you can think of it as three groups of three. The concept of three is doubly represented. The last, the Centaurs, is six, so that's two groups of three.
1. Muses - preside over song and prompt memory, children of Zeus and Mnemosyne (memory)
Calliope - epic poetry
Clio - history
Euterpe - lyric poetry
Melpomene - tragedy
Terpsichore - choral dance
Polyhymnia - sacred poetry
Urania - astronomy
Thalia - comedy
Erato - love poetry
2. Fates - determine future, people's destiny, children of Zeus and Themis (law)
Clotho - spins
Lachesis - weaves
Atropos - cuts it off
3. Furies (Erinyes, Eumenides) - punish those who escape or defy public justice
Alecto - never ceasing
Tisiphone - grudger
Megaera - avenger of blood
4. Graces - preside over banquets, dances, social settings, children of Zeus and Eurynome
5. Gorgons - hideously ugly monsters, snakey hair, can turn people to stone, the children of Phorkys and Keto, who in turn are the children of Pontos and Ge
6. Graeae (Graiai) - gray maids, old age personified, one eye and one tooth between them, the children of Phorkys and Keto, who in turn are the children of Pontos and Ge
7. Harpies - bird-like wind spirits, faces of women, bodies of birds, snatch people, children of Thaumas and Elektra, Thaumas is the son of Pontos and Ge
Aello - storm wind
Okypete - swift wind
Kelaino - dark
8. Cyclops - one eyed giants, the original ones although it seems that others existed later, children of Uranos and Gaea
9. Hekatoncheires - hundred handed giants, the logic was that they could throw a large number of boulders at once, also inspired by the octopus, children of Uranos and Gaea
Briareos (Obriareos, Aigaion)
10. Centaurs - half-horse half-men, known for lust and drunkenness, except for Chiron, who was a wise teacher, children of Ixion and Nephele
11. Oneiroi - control dreams, children of Hypnos and Pasithea
There are other examples of the repetition of three, such as Cerberus, who has three heads. Also, in Ancient Greece, they had trilogies of plays, such as the Theban trilogy by Sophocles, and the Oresteian trilogy by Aeschylus.