3 devices. icloud library
Yes. If you disable iCloud Photo Library on your iPhone, photos will no longer be uploaded or removed from iCloud as per your actions on your phone. The iCloud backup for your phone will update, but that obviously doesn't affect the other devices. 

Either way, it would be good to understand and have in place backup procedures. One is that the iPhone saves "deleted" photos for 39 days. After you delete photos from your iPhone I recommend that you check your iPad to make sure they aren't deleted there. Second, it would be good to backup all of your photos to a computer.