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)

Three dimensions
The Doctor's universe had three spatial dimensions, (TV: Flatline, AUDIO: Paradise Five) 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.