its hard to counter ap champs

So? They were FAR too easy counterable in season 2, you are not SUPPOSED to be able to just live through their damage as if it had been nothing as they have cooldowns that restrict them, as opposed to ad champs who can always auto attack.

Not really. Warmog's still prevents them from taking you out in one burst rotation more often than not, and while Runic Bulwark offers 16 less magic resistance to it's bearer than Force of Nature, it grants 30 magic resistance to all nearby allies. And auras are really nifty when you consider tanks aren't prime targets, which is generally the reason Frozen Heart was considered vastly supirior to Thornmail in season 2. Spirit Visage isn't a bad buy now either, and it increases your regeneration effects (including Warmog's) by 20%.

If you talk about folding magical resistance into your build as champions beyond tanks, you have even more options now in season 3. Carries and Bruisers can pick up Maw of Malmortis, Wit's End, and Mercurial Scimitar - granting solid magic resistance while still outputting reliable damage. Mages and Supports can pick up Abyssal Scepter, Mikeal's Crucible, Athene's Unholy Grail, and Twin Shadows. Both routes got more MR options this season rather than less, and you can add Guardian Angel and Mercury Treads on any build for an end-game build with plenty of magic resistance.