3 (number)

3 was a prime number. (AUDIO: The Haunting)

According to Jo Grant, three sevens were equal to 21. Also according to her, 12 divided by 4 was 3. (TV: The Claws of Axos)

Some planets had three suns, including one visited by the First Doctor, (TV: Galaxy 4) one visited by the Second Doctor, (PROSE: The Sour Note) the Planet of Light, (COMIC: World Without Night) Knosson, (PROSE: The Castaway) Telios, (PROSE: K9 and the Zeta Rescue) Justicia, (PROSE: The Monsters Inside) Murigen, (PROSE: Predating the Predators) San Helios (TV: Planet of the Dead) and Desolation. (TV: The Ghost Monument)

Hyph3n explained to the Thirteenth Doctor that her name was spelt with a 3 "after the second H and before the final N." (TV: Orphan 55)

Cathy Salt was three months pregnant when she met "Margaret Blaine". (TV: Boom Town)

Three dimensions
The Doctor's universe had three spatial dimensions, (TV: Flatline, AUDIO: Paradise 5) or "primary dimensions". (AUDIO: The Invasion of E-Space)

There were a total of eleven dimensions in all, (PROSE: Parasite) including either two dimensions of time, (PROSE: The Quantum Archangel) or time and space as the fourth and fifth dimensions. (TV: "An Unearthly Child") The remaining planes formed the Six-Fold-Realm. (PROSE: The Quantum Archangel)

The Boneless came from a universe with only two dimensions. (TV: Flatline)

Behind the scenes
Though unremarked in PROSE: Daisy Chain, 3 is also the fourth number in the Fibonacci sequence.