A quick man crontab will, hopefully, show you how to define the job in cron to run every three hours. For the actual script, creating the target filename would be like:
And you'd then use $tgt to refer to the target file name.
The moon on the one hand, the dawn on the other:
The moon is my sister, the dawn is my brother.
The moon on my left and the dawn on my right.
My brother, good morning: my sister, good night.
-- Hilaire Belloc