In short, “psychologism” for Husserl is the error of collapsing the normative or regulative discipline of logic down onto the merely descriptive discipline of psychology. It would make mental operations (such as combination) the source of their own regulation.
Husserl’s formulation and refutation of the three psychologistic premises is wickedly clever, but cannot be treated in detail here. (See # 43-49 of LU I.)
eidetisch 