The date-time that i have now:
"schedule": "R/2017-10-05T17:21:00/PT15M"
for now the job is scheduled for every 15 minutes (in chron), but if i want to perform it three times a day at a certain time and only Monday - Friday?
Is it possible to define in this format?
It's not possible
ISO8601 is designed to define intervals but only static. There is no way to define weekdays or weekends. This means you can only define interval for 3 times a day but not every day from Monday to Friday because then you will have not equal intervals between Friday and Monday.
What you can do is to create 15 jobs scheduled weekly.