They are three different years that are usually associated with the release or invention of android. Records do make clear the actual date android was invented.
Android at first was intended to be an advance operating system for digital cameras. After deliberation, the company decided digital cameras didn't have a large enough market base for it's goals.
Google in 2005 bought the rights of android from Android Inc (a small company at the time struggling to acquire investors) which was the initial developer. Google unveiled android to the general public in 2007 followed by the lunch of it's first commercial device in 2008. Drawing from this, it's quiet difficult to pick point an invented date fro android.
On the other hand, Android Inc which was the initial developer of android was founded in California by Andy Rubin, Mich Miner and two other persons in 2003.