What's more cost effective?

[Brucey19]

I have a Gigabyte EX58-UD5 with provisions for 3 video cards in PCIE-16 slots. Is it more cost effective if I buy 3 identical video cards with SLI, OR AMDS MATCH UP tech(forget what it's called). OR buy a video card that already has provisions for THREE monitors

[Haldi]

SLI or Crossfire are outdate and not widely supported anymore. The new thing (at least in games) is called DX12 MultiGPU Render. But it need's to be specifically supported by each game itself.

Getting one Card with 3 Video Outputs is the Better option in most cases. Especially if you want to use it for SLS (Single Large Surface) meaning all 3 Monitors spanned to a Single Desktop and maximize a game over all 3 Monitors.
Nowadays there is a thing called Displayport Daisychaining, you can use Multiple Monitors via a Single Displayport Cable IF you monitor possesses a Displayport OUT port and supports daisychaining.
But most Graphic cards nowadays have 3 Displayport and a HDMI Port or similar anyway.