The number three, used in counting, mathematics, and Sino-Japanese compounds. The reading サン is the on'yomi; the native Japanese reading み is not covered here.
from Chinese sān
三に二を足すと五になる。
Three plus two equals five.
このクラスには三つのグループがある。
There are three groups in this class.
From Middle Chinese 三 (sam). The reading サン is the regular Sino-Japanese on'yomi.