1. Some features may not be available in all countries or all areas. Flyover and turn-by-turn navigation will be available only on iPhone 4S and iPad 2 or later. Cellular data charges may apply.
2. Siri will be available only on iPhone 4S and iPad (3rd generation) and requires Internet access. Siri may not be available in all languages or all areas, and features may vary by area. Cellular data charges may apply.
3. Shared Photo Streams requires iOS 6 on iPhone 4 or later or iPad 2 or later, or a Mac computer with OS X Mountain Lion. An up-to-date browser is required for accessing shared photo streams on the web.
4. FaceTime over a cellular network requires iPhone 4 or later, or iPad 2 or later with cellular data capability. Carrier data charges may apply. FaceTime is not available in all countries.
5. VIP list and VIP and Flagged smart mailboxes will be available on iPhone 4 or later and iPad 2 or later.
6. Offline Reading List will be available on iPhone 4 or later and iPad 2 or later.
7. Made for iPhone hearing aids require iPhone 4S.
8. Find My Friends and Find My iPhone enable you to locate iOS devices only when they are on and connected to a registered Wi-Fi network or have an active data plan. Not available in all areas.
Source: Apple.com
So yeah. Don't know why 5/6 are not coming to the 3Gs but it's Apple, I guess.
2. Siri will be available only on iPhone 4S and iPad (3rd generation) and requires Internet access. Siri may not be available in all languages or all areas, and features may vary by area. Cellular data charges may apply.
3. Shared Photo Streams requires iOS 6 on iPhone 4 or later or iPad 2 or later, or a Mac computer with OS X Mountain Lion. An up-to-date browser is required for accessing shared photo streams on the web.
4. FaceTime over a cellular network requires iPhone 4 or later, or iPad 2 or later with cellular data capability. Carrier data charges may apply. FaceTime is not available in all countries.
5. VIP list and VIP and Flagged smart mailboxes will be available on iPhone 4 or later and iPad 2 or later.
6. Offline Reading List will be available on iPhone 4 or later and iPad 2 or later.
7. Made for iPhone hearing aids require iPhone 4S.
8. Find My Friends and Find My iPhone enable you to locate iOS devices only when they are on and connected to a registered Wi-Fi network or have an active data plan. Not available in all areas.
Source: Apple.com
So yeah. Don't know why 5/6 are not coming to the 3Gs but it's Apple, I guess.