Apple typically only hosts the absolute latest version on the Mac App Store . However, you can still obtain version 10.3.3 through the following official methods: Download imovie Version 10.3 - Apple Support Community
Use the Cinematic control in the inspector to modify depth effect intensity or shift focus between faces and objects. Imovie 10.3.3 Download
Approximately 2.4 GB to 4.3 GB of available storage. How to Download iMovie 10.3.3 Apple typically only hosts the absolute latest version