how to create a cron job that run in first day of the month , every 3 months.?
The following will run script on the 1st of Jan, Apr, Jul and Oct at 03:30
Alternatively, but less obvious
Will run every three months at 03:30 on the 1st of Jan,Apr,Jul and Oct.
Wikipedia has a nice explanation about how to configure Cron.
For your specific case you could run a Cron Expression to run every 3 months- obviously change the months to suit your schedule.