I used to own a iPhone 3GS and then moved on to the Lumia 900. From my own personal experience the Lumia was a nice upgrade. I was skeptical if the larger screen was a big deal or not. But now it feels really odd to go back to anything smaller. The Lumia 900 is 4.3". As far as OS goes. I really like the WP7.5. It is pretty smooth and is very intuitive once I was used to the change. There is still alot of unknowns as to what WP8 will have instore, but I've become convinced that the 920 will be my next phone. Apple sure does have a lot off apps. But I would say the 100,000+ on MS marketplace are pretty comparable. Though if you are a musician and like things like iRig and Amplitube, Apple seems to be the way to go right now. I've never owned and Android. Many are really happy with them. Many still complain about upgradeability. But those things are like every other ecosystem. For me, the 920 is the winner. Wireless charging, Rich Audio Recording, the best camera on the market, Nokia is exceptionally good at customer support constantly bring apps to the ecosystem, and I've really grown quite fond of Windows Phone. I'm hoping WP8 has all the flexibility Android claims to have with none of the issues or ROM flashing. But any of the 3 phones you chose will be nice.
Oh yeah, I left Apple because everything I use is MS. PC and want a Surface tab or something. iTunes is not great...nor is Zune, but Zune is being 86ed.
I suggest reading and playing. If you can find a WP in the wild, try it out...not at a store but an actual consumers phone.