I am struggling with "en" used with prepositional verbs. I understand that "en" replaces "de plus a noun", that is, en = (some/any) of (it/them) where the noun is animal, place, thing, idea, or quantity of people. I have been told - by a French speaker - that it is fine to say "J'en ai besoin de trois" as an alternative for "J'ai besoin de trois d'entre eux", that is "en" can be used here to represent "d'entre eux/of them", BUT that it is not OK to say J'en assiste à trois" as an alternative for "J'assiste à trois d'entre eux" where "en" again represents "of them". It seems that it is OK to use "en" with a verb which requires the preposition de, but that it is not OK to use "en" with a verb which uses the preposition à. Is this correct and if it is why is this so? Is there a grammatical reason for this or is it just down to common usage?