Multiple foreign keys on single column
It was decided to go with something between second and third options - adding additional table with columns Value type and separate columns for each Resource type.


(+) easier to manage in code
(+) availability of Foreign Keys on Resource ID
(+) no overhead on table count
(+) ability to add multiple different Resource type's to each Item
(-) uses extra space for each unused Resource type for each field
(-) lacks ability to cascade delete related Item, when resource with Resource ID is deleted.