i7Stealth1366, I don't think "best" is the term you're looking for, moreso maybe "budget" if a monitor over $200 each is making you question if you want to go with the setup or not.
In the same vein, while you did say "best", I see no point in recommending a BenQ XL2420T, which is a $400 monitor, while questioning monitors half that price.
Anyway, back to your questions. You will not need
tri-SLI for 3 monitor surround. It will surely help, because you're going to be pushing quite a bit of pixels, but not needed. You do need at least 2 card SLI though, that will enable nVidia surround. As amang said though, you are going to want cards with a decent amount of vram for best results. For nVidia's current generation of cards, I hear the 2.5GB GTX570's as well as the 3GB GTX580's do rather nice, but also come with a hefty price tag. Can it be done with something like a GTX560 ti SLI? Sure, but if you enable any eye candy in any games, you're going to take a performance hit. The reason for that being SLI only uses the vram of the first card, so even if you have 2x 1GB cards, you're only going to use 1GB of vram. and when you're gaming at 5760x1080, 1GB of vram doesn't go very far.
As far as monitors, there are some budget options out there depending on what screen size\resolution you're looking at, and what you expect to get out of it. I mean, you can get 3x Acer 23" 1080p
monitors for $450. Even more budget than that, 3x Acer 21.5" 1080p
monitors for $330. Neither of those will be the "best" like the $400 BenQ 120hz, but will give a good price\performance ratio for surround on a budget. From there, as you said, you can get decent 24" 1080p monitors for ~$200 each, and just go up from there depending on how much money you want to put into the project.
I've done quite a bit of research on this recently, as I'm making the same plunge next week, except I'm going the AMD route, since I'm not really a fan of multiple GPU setups. It's amazing though, how my original plan went from grabbing a used 6970 for ~$275 and 3x $200 24" 1080p monitors (didn't want to spend more than ~$900 originally), to saying "**** it, if I'm going to do it, might as well do it right the first time", and has ended at a 7970 for $600, and 3x Dell U2412M's for ~$330 each, capping out at close to $1600 (god bless tax returns).
Hopefully this info helps