Sample Code:
SchedulerClass obj = new SchedulerClass();
String cron = ‘0 30,30,30 8,12,15 * * ? *’;
System.schedule(‘SchedulerClass’, cron, obj);
In the above example, SchedulerClass will run daily at 8.30 AM, 12.30 PM and 3.30 PM.
To know more about scheduling, please check the below link
https://www.infallibletechie.com/2014/06/systemschedule-in-apex-scheduler-in.html
Cheers!!!