How to change gallery from 3 images to 1 image?

Hi there!

If you are talking about to lower/delete/edit the gallery images then you may follow the screenshots:

First please go to your sp property and find out the property item you want to edit the gallery for:
https://prnt.sc/1054pcu

Then open it up with edit mode:
https://prnt.sc/1054o7m

Then scroll down further within the basic tab and find the gallery options and do what you want:
https://prnt.sc/1054nse

Hope this helps!

Best Regards