Can a 3-pole circuit breaker be used in a single-phase application?
A 3p breaker can be used on a 3 phase system using either 2 or 3 poles.
A 3p breaker can be used on a 1 phase system only if indicated by the markings as noted below.
Per NEC article 240.85: "A circuit breaker with a straight voltage rating, such as 240V or 480V, shall be permitted to be applied in a circuit in which the nominal voltage between any two conductors does not exceed the circuit breaker’s voltage rating."
According to UL's White book, "3–pole circuit breakers are suitable for use on 3-phase systems only, unless marked to indicate use on 1-phase systems, such as, “For 1–phase connections, use two outside poles,” or an equivalent statement. A 3-pole breaker used in place of a 2-pole breaker on a 3-phase system, such as a 2-pole breaker used in a branch circuit that is actually two legs of a 3-phase system, is acceptable without the 3-pole breaker being specifically marked."