If the photos are in your Google Photos in the cloud, then your options are to manually download the photos to your tablet's local storage, or see if there's a way to sync your Samsung Gallery app with your Google Photos in the cloud. The latter has traditionally been done by turning the Picasa Sync option on in the Settings>Accounts menu, either in the Google account or Samsung account (this is in the system settings, not the settings for the gallery apps). If the Samsung Gallery app on the Tab S3 still allows you to sync with Picasa/Google Photos, then they should show up in the Gallery app with a little Picasa or Google Photos icon associated with the thumbnail.