Run it every three days - or less at the end of the month. (It'll run 2 days in a row if the previous month had 31 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.