That said, there are already tier 3 version of all three classes. Healer, Adept, and that Rich Burlew homebrew 'landtender' or whatever. Probly heaps more out there considering homebrew.
What? The Healer and Adept are both T5. The homebrew thing... do you mean the NPC class? That's probably on par with the Adept, without refreshing my memory. The Healer is just that, and the other two are NPC classes.
In other words, not what was asked for. Bortasz asked for Tier 2, not Tier 5 or even Tier 3.
As for answering the question, it's hard without changing the classes significantly. Each does have a Tier 2 pendant (Wizard/Sorcerer, Cleric/Favored Soul, Druid/Shaman), but those are all spontaneous casters, and for good reason - they're Tier 2 because they can reach the vertical power the Tier 1s attain, but are severely restricted in versatility in comparison.
Overall, though, I'm unsure of the sense this would have. 'Nerfing' those three classes down to Tier 2 doesn't do much to curb their game-breaking abilities - it just gives them less of those (think of it as a nation having fifty nukes versus a nation having two nukes).