Have a configuration but...

First off the build
Chassis Model: Special Deal Hot Seller - Black OPS Aventum
Exterior Finish: - Standard Factory Finish
Trim Accents: - Not Available
Processor: Intel Core i7 Extreme Edition 3970X 3.5GHz (Six-Core) (Extreme Performance)
Motherboard: ASUS Rampage IV Extreme X79 (Intel X79 Chipset) (Features USB 3.0 and SATA 6Gb/s)
System Memory: 64GB DDR3 1600MHz Corsair Dominator DHX (High-Performance)
Power Supply: 1500W Silverstone (Dual/Triple/Quad SLI Compatible) (Recommended)
Expansion Bay: Dual Solid State Hard Drive Hot Swap Bay
Hard Drive Set 1: Operating System: 1x (600GB Solid State (By: Intel) (Model: 320 Series)
Set 1 Raid Options: - No Thanks
Hard Drive Set 2: Multimedia\Data: 1x (3TB Western Digital/Seagate/Hitachi/Samsung (7200 RPM) (SATA 6Gbps)
Hard Drive Set 3: Backup\Misc.: 1x (3TB Western Digital/Seagate/Hitachi/Samsung (7200 RPM) (SATA 6Gbps)
Optical Drive 1: Blu-Ray Player/DVD Writer (Play Blu-Ray and Burn DVDs) (Slim Slot Loading Edition)
Optical Drive 2: - Not Available
Internet Access: Wireless USB 802.11n Up to 300Mbps (Supports 802.11n/g/b)
Video Card(s): 2x SLI Quad SLI (NVIDIA GeForce GTX 690 4GB (Includes PhysX)
Add-on Card: - No Thanks
Sound Card: Creative Labs Recon3D Fatal1ty Champion 5.1 (Includes Front I/O) (PCI-Express Slot Based)
Extreme Cooling: Digital Storm Aventum Cryo-TEC System for CPU and Liquid Cooling for Video Cards + Chipset
H20 Tube Color: Black Tubing with High-Performance Fluid (UV Lighting Reactive)
Chassis Airflow: Digital Storm Aventum Automatic Response to Excess-Heat System (ARES)
Internal Lighting: Digital Storm Aventum RGB LED Lighting System
Enhancements: Digital Storm Aventum Integrated Side Panel Display Module
Chassis Mods: - No Thanks
Noise Reduction: - No Thanks
LaserMark: - No Thanks
CPU Boost: Stage 2: Overclock CPU 4.5GHz to 4.8GHz (Requires Pro/Deluxe/Sabertooth Motherboard)
Graphics Boost: Yes, Overclock the video card(s) as much as possible with complete stability
Memory Boost: - No Thanks, Please do not overclock my memory
OS Boost: FREE: Yes, Disable and tweak all of the non-crucial services on the operating system
Windows OS: Microsoft Windows 7 Ultimate (64-Bit Edition)
Recovery Tools: Windows Recovery Toolkit (Bundled with Windows CD)
Virus Protection: - No Thanks
Office: Microsoft Office 2010 Pro (Full Retail Box Packaging)
Game: - No Thanks
Display: Acer 27 inch (Widescreen) (Black) (High-Performance Gaming Display)
Surge Shield: APC BX1500G Uninterruptible Backup Surge Conditioner (Recommended)
Speakers: - No Thanks
Keyboard: Corsair Vengeance K90 (MMO Mechanical Gaming Keyboard) (Black/Metal Finish)
Mouse: Corsair Vengeance M90 (Laser 5700 dpi) (MMO/RTS Gaming Mouse) (USB Wired)
External Storage: - No Thanks
Exclusive T-Shirt: FREE: Digital Storm T-Shirt - Black (Extra Large)
Priority Build: - No Thanks, Ship Within 10-15 Business Days After Order Is Successfully Processed
Warranty: Life-time Expert Customer Care with 6 Year Limited Warranty

Any Comments are welcome. But what i want to know is what do i have to do to get them to add a third NVIDIA GeForce GTX 690 4GB for 3x sli.

Thanks.

bprat22 is right...as usual...
Remember the GTX 690 4 gb card is actually equal to 2 x GTX 680 2 gb cards.    More gpu's does not equate to better graphics after a point. If the 4th GPU is wasted, why consider more anyway? Perhaps if you explain the useage planned...This quote I saw in another forum sums up why 3 x 680 4 gb would be preferred over 2 x 690 4 gb.

"3-way SLI is in fact more efficient than quad-SLI. And no, I'm not talking about the worthless synthetic benchmarks. I'm talking about real world in-game performance. It's a fact that the fourth GPU in the quad-SLI setup just doesn't get utilized as one might think and overall scaling is horrible. This is a similary issue with 4-way SLI. The fourth card is pretty much useless except for running dedicated Physx. The limitations of quad-SLI setups been in existence since dual-GPU cards were introduced. 3-way SLI has always performed better and provided much more stable performance in almost all games. Also, with a quad-SLI setup you have to deal with the microstuttering issues. 3-way SLI does not have this problem. Another point to make is that with a quad-SLI setup, the total VRAM is limited and is not ideal for surround gaming where the VRAM buffer fills up quickly up to and past 2GB. Remember, the 4GB of DDR5 on the GTX690 has to be shared between the two GPUs, resulting in only 2GB for each."   

You sir may need a NVidia Workstation professional card like the Quadro... Quadro cards can do up to eight displays. Run it by Alex... He will get you the answer from Digital Storm...
[email protected]
NVidia SLI is limited to 3 active displays in surround and one reference display.... For 6 displays with GeForce desktop cards you would need to disable SLI and can use 3 680's to plug 2 displays into each card, and extend the desktop.... or 2 680's with 3 displays each.

Also, I believe AMD Eyefinity using a HD 7970 for example will display across 6 monitors.  It's 3 Gb vram would be better than the actual  2Gb of the gtx690 anyways.