Both iOS 10 and watchOS 3 will be released to the public on Tuesday, September 13, three days before the September 16 release date of the iPhone 7 and the iPhone 7 Plus. The iPhone 7 and the iPhone 7 Plus will both ship with iOS 10 while the Apple Watch Series 2 will ship with watchOS 3.
macOS Sierra will be released to the public one week later, on Tuesday, September 20, according to Apple's macOS Sierra webpage.
Apple has not yet provided a release date for the tvOS 10 operating system that has also been in beta testing since June. The tvOS 10 website simply says "Coming Soon."