Hi David,
To let one grid's selection determine another grid's contents, you should place the second grid inside a data view that listens to the first grid.
In your case, the grid on Ticket_Type should be placed inside a data view on Ticket_Category. This data view should listen to the grid on Ticket_Category using the 'Listen target' property. For the grid on Ticket_Type, you can use either a path from the Ticket_Category data view or an XPath constraint to constrain the contents based on the selected Ticket_Category object.
The same holds for the grid on Ticket_Item. It should be placed in a data view on Ticket_Type, which listens to the grid on Ticket_Type.
Hope this helps, let me know if any of this is unclear!