iPhone 3GS launched with iOS 3.0, which added basic features like cut, copy, and paste or MMS support. Then it was updated to iOS 4.0, which supported third-party apps multitasking. iOS 5.0 brought iCloud and better notifications, while iOS 6.0 for 3GS was a limited release that didn't include many features that were available for iPhone 4S/5. Apple got the foundation right and started to add features later, while Google started with the features and fixed the foundation later.
Back in 2009, Android was buggy and slow, so Google released new software updates quite frequently. Android 1.1, 1.5 and 1.6 added new features and fixed bugs, but didn't focus on the user interface. At that time, not many Android phones were available: G1, HTC Dream, HTC Hero, but Motorola Droid and Verizon's ad campaigns put Android on the map. Motorola Droid had better specs than iPhone 3GS: higher resolution screen, bigger battery, hardware keyboard, microSD support, multitasking, free turn-by-turn navigation. Droid
In January 2010, Google launched Nexus One, a phone with much better specs than Motorola Droid: 1 GHz Qualcomm Scorpion and 512 MB of RAM. It was updated to Froyo and Gingerbread, but the limited internal storage was the most important flaw that made it obsolete. In fact, the only Android phone released in 2010 that was officially updated to Ice Cream Sandwich is Nexus S (December 2010). Ice Cream Sandwich was launched in October 2011.
Meanwhile, Apple's iPhone 3GS is the only iOS device that received 3 major OS updates. Apple usually supports iOS devices for 2 years, so you get at least one major OS update and most often 2 updates. 3GS was still sold (free with contract in the US) until September 2012, when iPhone 5 was announced. 4 years of software updates is unprecedented in the mobile world, with hardware and software that evolves so quickly.