Professional reference could be from someone you worked with. In any job. Don’t need to be your boss. Or professional job or full time job. Simply a coworker. Past or present.

Academic reference is acceptable as a professional reference. These have to be people who know you in a setting that is somewhat professional but doesn’t always mean work (students who just graduated maybe didn’t ever work yet). Since you have several degrees you have met ton of professors. You can use any of them, all three could be college professors.

Also even if you didn’t have long term or full time employment you did have some type of jobs, anyone you worked with at any point could be a reference. You could find people on social media if you lost contact.

My husband got a new job last year and he really wanted his former DON to be one of the references, he couldn’t find him as guy moved out of state and changed phone number. Sure enough he found him on Facebook and the guy was more than happy to be a reference. So these people could be from your old jobs from years ago