I know this is a few months old but I recently had this same problem. The gear S3 classic will scratch, and is especially noticeable on the polished part of the watch case where the watch band attaches, but it has to scrape against something fairly hard. Luckily if you do scratch that area of the S3 Classic, you can buff it out safely.
Do this ONLY for the S3 Classic! :
You need to get some small pieces of masking tape to tape off the brushed side of the watch and tape off the bezel as well.
You should only have the part you want polished exposed. Next you want to get "Mothers Mag and Aluminum polish" and a Q-tip.
Get a very very light coating on your Q-tip and lightly rub where the scratch is several times. You only need slight pressure.
Make a few passes then take a soft clean cloth and wipe the polish off to see your progress.
You may need to make a few more passes till the scratch is completely buffed out.
After your final passes, wipe off the polish, take off the tape and buff clean with a soft cloth.
If you are very careful this should work perfectly and make your Gear S3 classic look just as good as the day you pulled it out of the box!