Low pressure 3" shells can be safely shot in a 2 3/4" chamber just as can low pressure 2 3/4" shells in a 2 1/2" chamber. Full pressure 3" shells should not be shot in a 2 3/4" chamber just as full pressure 2 3/4" shells should not be shot in a 2 1/2" chamber. That is not having it both ways, simply having it as it is.
It is the nature of most propellants, certainly the ones used in firearms, that the higher the pressure, the more sensitive they are to factors that raise pressure. The relationsips are non-linear. A low pressure shell would likely have a very small pressure rise when fired in a short chamber. On the other hand, a shell loaded to near maximum may have a significant pressure rise due to being fired in a short chamber.
