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
Samsung Original Replacement Battery Eb-br760abe For Samsung Gear S3 Frontier / Classic Smart Watch Sm-r760 Sm-r770 Sm-r765 - Mobile Phone Batteries - AliExpress
Replacement Battery EB-BR730ABE For Samsung Galaxy Gear Sport Smart Watch SM-R600 Tool - Walmart.com