Feedback Form Enhancement Needed

I have a feedback form at [login to view URL] that allows people to upload photos. Uploading photos has been optional up until now. I would now like to make it mandatory that at least 3 photos are uploaded. When someone tries to click the SUBMIT button without uploading any photos, I would like for a message to appear that says, “You must upload at least 3 photos in order to receive a diagnosis.” I’ve attached the PHP page so you can see the code currently in use. If you need any other related files, please let me know.