Ciao
you can try this suggestion:
cron jobs on vcsa 6.7 – Nick's Container on the cloud 
(ntitta.in)
vCenter Server Appliance 6.5 stops sending alert emails (54526) (vmware.com)
and for schedule every 3 months you can use on this crontab combination on file .cron
30 03 01 */3 *
