What Happened to the Cron Options?
The new version of MailPoet no longer relies on the cron system shipped with WordPress. That system was responsible for starting the sending process and ensuring that your emails are dispatched according to the set schedule. However, it had its quirks and most often than messages were late to send.
Instead, MailPoet 3 offers a much faster and more reliable method for sending your newsletters on schedule. You may wish to read more about the new Newsletter Task Scheduler and how to use it.