A standard orchestral trumpet usually has three valves, which change the length of its tubing and produce different notes.
Players combine the valves in seven basic combinations, including using no valves. Each valve lowers the instrument’s pitch by routing air through additional tubing. Skilled players also adjust pitch with their embouchure and the movement of their lips.
Most B-flat trumpets use three piston valves. Some specialized trumpets, including certain piccolo trumpets, may have four valves to improve range and intonation. That variation explains why “three” describes the usual standard rather than every trumpet ever made.
Before valves became common, natural trumpets relied heavily on the harmonic series and could not easily play every chromatic note. The valve system made modern orchestral and band trumpet writing far more flexible.