How are item effects applied?

If I have a ring that gives -5% spell cost, an amulet that gives -10% spell cost, and robes that give -20% spell cost am I getting a full 35% off spell cost or are they all applied separately?

As an example, if a spell costs 100 magicka getting 35% off of that would leave it costing 65 magicka.

But if the effects are applied one after another the same 100 magicka spell would cost 20% off of 100 which is 80 then 10% off of that 80 leaves you with 72 and then applying the final 5% off of 72 leaves the final cost at 68.3.

Are item buffs added and then applied or applied separately?

The effects are added, and then applied. So if you were to enchant four items (head, neck, chest, ring) with a -25% Fortify Destruction enchant, you could cast any destruction spell without draining your magicka at all!

25% is the highest you can reduce the spell cost with a single enchant, here are some instructions on how to accomplish this:

http://www.reddit.com/r/skyrim/comments/mjulb/0_magicka_cost_spells_fully_explained/