You can get a system, which won't lose follow in cities, even Dalaran.
It will be playable everywhere, but not necessarily smooth.
Cutting the graphic settings is a good idea for this, in particular view distance, shadows and weather effects.

A quad core system, Q6600 overclocked to 3.0 ghz
With 4-6gb of ram.
A graphics card which was new in the last 2-3 years.
That will run 3 accounts, playable.

For three accounts, smooth everywhere.
I'd assume you're looking at an i5 or i7 system.
6 GB of ram will be plenty, for 3 accounts.
A low-end SSD, purely for the gaming folder.
A 64-bit OS, probably Win7 Home Premium.