  Uncertainty relations provide fundamental limits on what can be said about
the properties of quantum systems. For a quantum particle, the commutation
relation of position and momentum observables entails Heisenberg's uncertainty
relation. A third observable is presented which satisfies canonical commutation
relations with both position and momentum. The resulting triple of pairwise
canonical observables gives rise to a Heisenberg-type uncertainty relation for
the product of three standard deviations. We derive the smallest possible value
of this bound and determine the specific squeezed state which saturates the
triple uncertainty relation. Quantum optical experiments are proposed to verify
our findings.
