Personally I would purchase 1 fairly decent computer and 2 average computers. For 5K you could easily pull it off. Have 3(or 4) monitors, 2 party members on each side computer with its own monitor and ofcause your main on the centre monitor. You could put a second monitor on your main PC for browsing / DVD's and such.

While a tri-SLI system would be "nice" its not really suitable for WoW as WoW does not support SLI and if you are using it in SLI mode then you only have a single display. You can't use multiple displays when you are running in SLI mode. But if you are not in SLI mode then you have 6 display outputs available for use. Assuming you have 2 heads per graphics card.

Five k is more than enough to go for either option.