I can try...
Two ways:
- the easy way, because it doesn't strictly block access to the documents themselves: exclude the documents from the views they have, using a selection formula like
Form="YourForm" & FieldC!="Category3"
You need special views for yourself to see all documents
- the harder way: add Readers fields to all documents; use the Category as a role in the Readers field; assign a role to every user, so most users would have roles "Cat1"-"Cat5" except "Cat3"; if you do it like this you don't need additional views and documents are really hidden from them