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
430mAh Reaplacement Battery for Samsung Gear S3 Frontier (SM-R760, R770, R765), Gear S3 Classic Replacement Battery EB-BR760ABE GH43-04699A with Professional Repair Tools Kits : Amazon.ca: Electronics