Hi David,
From your description, you want the column A and B always be displayed when the report run and the column C to be hidden when the report initially run and can be toggled by the column B, right? In this case, we can drag the column C to the tablix, and then
add row group for it, the parent group is column B, and then insert a column in the left of the column B inside the group, display column A in the new inserted column. Then right click the Details in the Row Groups pane and select Group Properties to set its
can be toggled by column B. The screenshots below are for your reference.


Reference:Add a Visibility Toggle to an Item (Reporting Services)
Hope this helps.
Regards,
Charlie Liao
If you have any feedback on our support, please click
here.
Charlie Liao
TechNet Community Support
