Your linked Google Images search leads to examples such as this:
The manufacturer being Mechanical (the company's name). A little dive into the patents reveals this invention:
Source: Google Patents US3145281A
Also by Mechanical, for, you guessed it, a Multipole circuit breaker with trip devices located in the housing of a single pole. ...
A three-phase electric motor can in some cases continue to run on only two phases but at reduced power, depending on the nature of the load it is driving. Having separate breakers for each phase would allow this.