What does 'in a third' mean?

Prison staff said a lack of resources meant that only two out of their 15 guard towers were occupied at any given time. They told a local newspaper they put a football with a prison officer's cap in a third, to create the illusion of an extra guard

They had a football with a prison officer's cap in a third to create the illusion of an extra guard.

The 'in a third ' thing bugs me a lot. What does it mean?

Thank you!