To save as an MP3 is simple, go to...

FILE > Export > as MP3

The real question is...what exactly do you mean by, "save my work..."

The only reasons to export as an MP3 (or any other format) is to...

1. Export the FINISHED project to a usable format.

2. To create an unfinished "take" for others to listen to and offer their opinion.

3. To export part, or all, of a project to import as an element in another (or the same) project.

"Saving" a project as an MP3 will NOT allow you to re-import it and edit it any further.  You need the original .mmm, .HDP, .H0, and media files for that.