Q: What does a quick change in status from "With Editor" to "Under Review" mean?

The status "Under review" is often used by journals to refer to both the internal editorial screening or external peer review. Depending on the average time for first decision, this status change could mean either of the following:

1. The paper has been assigned to an Associate Editor (AE) who is undertaking an internal review of your paper (also known as initial screening) to decide whether the paper will be sent out for peer review or desk rejected.

2. The Associate Editor (AE) has completed the initial screening, sent out reviewer invitations, and  the invitations have been accepted, that is, your paper has been sent out for external peer review.

Unless this is a rapid publication journal, the second option seems unlikely, since three days is too short a time for the editor to complete all of the tasks mentioned. In my opinion, the first option has a higher possibility. Thus, your paper is probably with the editor now undergoing the initial screening process. The next status change will make things clearer.

Related reading:

• What does the "With Editor" status mean for Elsevier journals?
• Why does the status date change while the status remains 'With Editor'?