The Apple Watch Series 3 makes use of the T8004 (S3) Processor. It also contains a GPS module, and on cellular models an LTE chip.
Specifications
Display: Retina OLED
Screen: Ion-X glass or sapphire crystal
Compatibility (GPS): iOS 11 or later running on an iPhone 5s or newer
Compatibility (GPS+LTE): iOS 11 or later running on an iPhone 6 or newer