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
China Vedor Smartwatch Battery for Samsung Gear S3 Frontier Classic Eb-Br760abe Br760 - China Samsung Gear S3, Watch Replacement Battery | Made-in-China.com