It will help lower 3-4 degrees, but really the backplate is for looks. What's going to help is a good case with plenty of cooling. You can always change the TIM on the cards and that will really help lower the temps. Good luck with your build. Make sure you go over to the Mod Rigs section and show them pics.

Make sure your 580s didn't come with high flows before you order them.  My second 580 came with a high flow already installed. 
 
As others have said, the back plate is mainly for looks (and it looks good), and might lower your temps slightly.
 
I've seen people say that a 1200w power supply is fine for 3 way 580s.

biggoliath

I only build a new system every 5 years or so.  The EVGA forums are incredible when it's time to play catch-up on what's new.  I went ahead and pulled the trigger on the P67 series.  Many components are on their way and some are sitting right here in waiting, including 3 x GTX 580 SCs.  I hope the Antec 1200w TPQ will power them well.

Since I will be using air cooling would any of you recommend grabbing 3 x high air flow brackets and back plates?  I guess I would like to know how effective they are.

Thanks for your time and thanks EVGA for a great resource.

The backplates only cool it by a few degrees. At first i had 2x 580's and decided it would be interesting to get another and see the performance boosts i got. 3-way SLI scales surprisingly well but with 3x 580's packed like sardines, the temperatures were a lot higher than 2x 580's plus 3 of em made the system louder too with that extra fan.
 
I decided it would be a good idea to sell all my 580's and buy 2x 590's. One coz there would only be 2 fans and a gap between the PCI-E meant cooler and quieter. Plus they'd be running at 16x/16x. Seemed like a great idea.
 
In truth though, after seeing how lame Quad SLI scaling is and seeing a lot of microstutter in benchmarks and in some games i'm a little dubious about ever going quad SLI again now. In hindsight, 2x 580's were perfect. They had enough performance and 2-way SLI scaling is absolutely perfect.
 
I think what i'll do is keep these 590's until the high end Kepler cards come out then get 2x 680's (if they call em that). Coz in every generation so far, two of the best single GPU cards seems to hit the sweet spot every time.

Highflow brackets could be included already in 580SC´s.
Backplates are for look.
And together you get max 2C drop. Maybe more worth with tightly packed 3-SLi. But make sure you have a good airflow into case.
And if you go with 3 x 580´s, then get the 2600K for CPU.
 
I think your PSU is enough for setup. It shows like giving a good amps there.
 

I Bought 3 x GTX 580 SCs got them tuesday.
 

Definitely HAF X!!!! Switching from HAF 922 to HAF X resulted in two things: utterly quiet system (for MY ears though) and drop in temp; both PSU as well as GPUs!!! High flow definitely helps but I wouldn't count in on backplates to drop temps (looks good though). Especially for tri SLI, I would suggest you to use the Gpu airflow brackets that comes with hafx. You've got to place this behind the GPU and can fit a 120mm fan blowing 'push' air towards the GPUs. Dunno how to explain, best is let me upload couple of pics of my new case:


a bit close:

quality could be poor as it's uploaded via mobile phone.

Good luck!!!

mudvari your rig is creeping me out!  We have very similar builds.  I wish I had that megashadow cooler! 
 
OP the 1200W PSU is just fine and the HAF cases are some of the best for air cooling.  I replaced the TIM on a couple of my cards with ICD and I can report no drastic changes in temps at all!  If you ordered those cards recently they will have the high flow brackets.  

kmconstable

mudvari your rig is creeping me out!  We have very similar builds.  I wish I had that megashadow cooler! 

OP the 1200W PSU is just fine and the HAF cases are some of the best for air cooling.  I replaced the TIM on a couple of my cards with ICD and I can report no drastic changes in temps at all!  If you ordered those cards recently they will have the high flow brackets.  

Lol...didn't notice this!! And I am adding a sound card, which too is similar to yours!! Well not exactly performance-wise but name-wise; slightly!! Bought a sound blaster titanium PCIe and a Roccat Kave 5.1!!!! I love the mega series coz of their easy mounting method!! If you want one you should get one ;)

@OP: as junglekorak mentioned I would suggest you to use the ICD as well!! Worst case scenario u will end up with similar temps with and extra knowledge of 're-timing GPUs' ;). Although I noticed that OCZ freeze cools the CPU better (correct me if im wrong) and icd cools Gpu much better!!