Sounds arkward, i do run my W3520 @4.2ghz @ 1,28v with a triple rad too and mobo is just on stock cooling, no problems so far
As for going 3-way, that will be a lot of heat, not to think about when the cards are overclocked. Rads can do this much heat, but with very strong fans -> very loud. For a test you can put the i7 and two gfx cards in one loop and see if it's working, if it is, 3-way should do aswell. Something external like a Mora (9x120) would do great with this, but i think that's not an option since you've build everything up so far. Maybe another dual or triple in the ceiling ?