This kanji is a pictograph of three horizontal lines representing the number three. It's one of the simplest characters, visually demonstrating its meaning.
'San' — like the three arches of a 'SAN'd dune.
Vocabulary
三つみっつ
three (general counter)
三日みっか
the third day of the month, three days
三回さんかい
three times
三個さんこ
three items (general counter for small, distinct objects)