Automated outreach will email one contact at a time. It will cycle through your first, second, and third attempts, waiting the specified number of days before each follow-up attempt, to the first contact in line. If you do not receive a reply after the sequence has completed, it will move on to the next contact and repeat the same process.
So, the sequence will go like this:
Contact A: 1st Attempt
-- wait x days
Contact A: 2nd Attempt
-- wait x days
Contact A: 3rd Attempt
-- wait x days
If no response from Contact A:
Contact B: 1st Attempt
-- wait x days
Contact B: 2nd Attempt
-- wait x days
Contact B: 3rd Attempt
...
Contacts are emailed in the order that they are arranged on the Personalize stage of the Workflow. If you want to change this order at a later date, you can rearrange the contacts from the Opportunity Details page. Scroll to the 'Contact Info' box and pressing 'Reorder'. Contacts at the top of the list will take priority.