w.t.f? The iPhone 3G started with iOS 2, and was supported up to iOS 4.2.1, which is 2 years.
What did you do, pick one out of the garbage bin when Apple stopped selling them?
And, if you look at https://en.wikipedia.org/wiki/List_of_iOS_devices, you might notice that it's getting longer.
The 3GS and 4 went up to 3 years, and the 4S is currently at 4 years [it's the oldest device supported by the current version of iOS].
Google doesn't even support their own Nexus devices this long.