You can run 3 on a dual core, but if you're going for a new and very inexpensive machine, it'll be tough to do better than something like this for the price:
Intel Q6600 $195
Asus P5Q-E $160
4GB (2x2GB) Corsair DDR2-800 $87 (or double it to 8GB for $174)
Asus HD 4850 $188 (or two of them for $376)
Corsair (SeaSonic) 520W PSU $115 (or SeaSonic M12 700W PSU $170 if overclocking..)
500GB Samsung F1 $70 (Lot of other options here.. depending on how much you want to spend)
$815

The main suggestions I can offer..
If you have two displays, get two videocards and power each to their own display without SLI/Crossfire.
To reduce lag in high-texture areas (Shatt/IF/Og), get a high-speed hard drive (WD Velociraptor 300GB $280) and sym-link wow data directories.

However, 3 characters can be easily done on an older E6xxx processor, 4gb of memory, and one 8800GT/S videocard. It's really all about how much you want to spend..

Assuming you want to spend ~$1500, I'd go for this:
Q9450 $320
Asus P5Q-E $160
8GB (4x2GB) Corsair DDR2-800 RAM $174
2x Asus HD4850 $376
SeaSonic M12 700W $170
300GB WD VelociRaptor HDD $290 (or get a 150GB WD Raptor & 750GB Samsung F1 for a total of $270)
$1490.

You can always add more larger & slower hard drives later.. Samsung F1 750GB drives are now $100 each.