There are good reasons Photoshop introduced the three file saving options. I think it can be summarized as follows. "Save" updates the open file with all edits. "Save As" allows the user to save a new version of the file in any format that can accommodate all the edits, while preserving the original version. "Save a copy" allows saving a new copy of the file in any available format including formats that are not able to accommodate all the edits, while preserving the original version.
Since this caused lots of confusion and complaints (e.g. this thread), Photoshop added an option to revert to the legacy "Save As". Open Preferences > File Handling > then check the box labeled "Enable legacy "Save As"".
