Just replaced my Gear S3 battery (24$ + 10$ for tools). Excited to get 3+ days back. Links in comments, will report back with new battery life in a few days. : r/GearS3
Amazon.com: Gear S3 Frontier Battery, MAXBEAR 450mAh Replacement Battery EB-BR760ABE for Samsung Gear S3 Frontier(SM-R760) and Gear S3 Classic,SM-R770,BR760,R765,GH43-04699A Battery with Repair Tool Kit : Cell Phones & Accessories