I am a little confused about the notations. What does Symbian^3 mean? Also, how does Symbian^x relate to Symbian OS v9.y ?
Symbian^x != Symbian OS 9.y The Symbian^x releases are built upon the original Symbian OS 9.4 release plus Nokia S60 layers. Since the base OS continues to evolve, though, its not useful to think in terms of the old 9.x numbering - we've moved beyond those now. What is worth noting, though, is that in general Symbian^x releases are backwards compatible with Symbian OS 9.y releases (though there are a few breaks - see the release notes for details).
Symbian^1 is the equivalent of Symbian OS 9.4 plus Nokia S60 5th edition layers (http://developer.symbian.org/wiki/index.php/Symbian%5E1)
Symbian^2 is Symbian^1 plus a few fairly minor features (http://developer.symbian.org/wiki/index.php/Symbian%5E2#Introduction)
Symbian^3 is focussed on UI and architecture improvements (http://developer.symbian.org/wiki/index.php/Symbian%5E3_Developer_Overview)