You can view a 3-phase breaker as group of three (3) single phase breakers. For example, a 30A 3-pole breaker consists of three 30A single phase breakers operating in tandem. So each "pole" of of a 3 phase breaker is a single phase breaker. Since only one pole will be serving the single phase load, the size (ampere rating) of a 3 phase breaker will remain the same as that of a single phase breaker.
This is not to condone the use of a 3 phase breaker for a isolated single phase load.
If this does not help, you need to clairfy your question further.
