3

There are 3 letters in an IATA airport code (worldwide, IATA). The three-letter codes on a boarding pass. They are commercial identifiers and need not resemble the city name, which is why Chicago O'Hare is ORD, after the orchard the field replaced.

There are 3 channels in the RGB colour model.

There are 3 wires in a domestic mains plug (United Kingdom and most of Europe). Live, neutral and earth. The earth wire carries no current in normal operation and exists entirely for the fault case, which is the clearest everyday example of designing for what should never happen.

The diameter of the Moon is about 3 thousand kilometres. 3,475 kilometres, roughly a quarter of the Earth's width — unusually large next to its planet, which is why the two are sometimes described as a double system.

Commonly, there are 3 items in the grouping people spontaneously use when memorising digits (digit-span studies). Chunking into threes is why phone numbers and card numbers are written in groups rather than as one run.

There are 3 bones in the middle ear, the smallest in the body (humans). The stapes is about three millimetres long. All three evolved from bones that were part of the jaw in earlier vertebrates.

There are 3 hearts in an octopus (octopuses). Two pump blood through the gills and one to the rest of the body; the systemic heart stops when the animal swims, which is part of why octopuses prefer to crawl.

There are 3 feet in a yard (the imperial and US customary systems). The yard is exactly 0.9144 metres by the 1959 international agreement, so the foot is exactly 0.3048 metres — the metric values are exact and the traditional ones are what they were rounded from.

There are 3 toes on each foot of a sloth, in the three-toed genus (Bradypus; two-toed sloths have two on the forelimbs). The names are misleading: all sloths have three toes on the hind feet, and the distinction is in the front limbs. The two groups are not closely related and evolved their slowness separately.

There are roughly 3 million sweat glands in the human skin (adults). Counts range from about two to four million between individuals. Humans sweat more efficiently than any other large mammal, which is the physiological basis for endurance running.

There are roughly 3 trillion trees on Earth. Figure as of 2015. A 2015 satellite and ground-survey estimate that revised the previous figure upward by a factor of about eight.

There are roughly 3 billion base pairs in the human genome (humans, one copy of the genome). Quoted between about 3.1 and 3.2 billion, depending on whether the count is the reference sequence or a complete gapless assembly. Printed at ordinary book density it would run to about a million pages.

There are 3 bases in a codon (almost all life). Two bases would give only sixteen combinations, too few for twenty amino acids; three gives sixty-four. It is also why a single insertion is so damaging — everything after it is read in the wrong frame.

3 is the atomic number of lithium, the lightest metal, used widely in rechargeable batteries.

Three is the conventional number of chances given before a consequence follows, in sport, in disciplinary procedure and in software retry logic. The choice is convention rather than calculation.

Three beats to the bar gives triple time, the metre of the waltz and the minuet. It feels different from four because the emphasis lands on a repeating group of three, producing the characteristic lilt rather than a march.

The rule of three describes a rhetorical and narrative pattern in which three items are more satisfying than two or four. It is a widely observed convention of composition rather than a property of the number.

In Korean custom, it is traditional that three is treated as an auspicious number, and things are offered or repeated in threes (Korea). It appears in the three-part structure of traditional ancestral rites and in the folk preference for doing a thing three times to make it count. Distinct from the East Asian homophone taboos, which turn on how a word sounds rather than on structure. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

Western numerology assigns three the meaning of creativity and communication in a life-path reading. Presented within the practice as ancient. The life-path calculation it comes from — reducing a birth date to a single digit — is documented from the early twentieth century, and no earlier source uses it. This is a modern popular interpretation, not a historical or doctrinal claim.

Presented as belief or tradition, not as an established fact.

Several traditions organise central doctrine in threes: the Christian Trinity of Father, Son and Holy Spirit; the Hindu Trimurti of Brahma, Vishnu and Shiva; and the Three Jewels of Buddhism. These are structurally similar groupings rather than the same idea, and each carries its own distinct theology.

In Christianity, God is confessed as three persons in one being, the Trinity. Formulated at the councils of Nicaea in 325 and Constantinople in 381. The doctrine is what distinguishes Trinitarian Christianity from Unitarian and non-Trinitarian traditions, so the number is a dividing line rather than a decoration.

Presented as belief or tradition, not as an established fact.

In Buddhism, taking refuge in three things — the Buddha, the teaching and the community — is the formal act by which someone becomes a Buddhist.

Presented as belief or tradition, not as an established fact.

There are 3 letters in an ISO 4217 currency code (worldwide, ISO 4217). The first two are usually the country's ISO code and the third the currency's initial — GBP is Great Britain plus pound. The rule breaks for currencies shared across countries, which is why the euro is EUR.

Three points not in a line define a plane, and three sides are the fewest that can enclose one — which makes the triangle the simplest polygon and the reason triangular bracing is used to make structures rigid.

In cricket, football (soccer), ice hockey, and several other team sports, a player scoring three goals, wickets, or equivalent feats in a single match achieves a 'hat-trick' — one of the most widely recognised individual achievements in team sport.