Run it every three days...
0 0 */3 * *
How about that?
If you want it to run on specific days of the month, like the 1st, 4th, 7th, etc... then you can just have a conditional in your script that checks for the current day of the month.
if (((date('j') - 1) % 3))
exit();
or, as @mario points out, you can use date('k') to get the day of the year instead of doing it based on the day of the month.
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