can i relate an entity with three other entities(weak) over an identifying relationship. For example, an entity item is related to three other entities book_item,cd_item and magazine_item over a "is a" relationship.(book_item,cd_item and magazine_item are weak entities)is this possible ?
While subtyping can be represented as an identifying relationship without a weak key in classic ER diagrams, it's best to distinguish the concepts. A subtype is a subset, meaning identity isn't changed in the process. A subtype of a regular entity set is still regular, a subtype of a weak entity set is weak.
This is different from parent/child entity sets wherein children are identified via the parent identifier and a weak key. Weak children are a different entity set from their parents, whereas subtypes are a subset of the same entity set.
I suggest you use one of the subtyping notations of EER diagrams, and don't mix identifying/weak terminology with subtyping/supertyping, unless for an academic exercise in classic ER.