If you're powering all the leds that could be connected to 3 32port LEDWizs, at, say 20ma per LED (conservative), that's 32*3*20, or 1920ma, or almost 2 amps of current.
That's no small amount of current.
Keep in mind that a USB port is specd to deliver +no more than+ 500ma, period. And if it's not a powered hub, it'll only deliver a fraction of that before it either crowbars and shuts down, or worse. 500 ma, using 20ma LEDs amounts to 25 LEDs at the absolute maximum, and that's not counting the current draw from the LEDWiz itself.
Bottom line, you just about HAVE to make use of the external power connectors on the LEDWiz, esp if you ever expect to light most of the LED's simultaneously.
You might get away with not doing that if you only light a few LEDs at a time, but that can be pretty tricky.
When you do grab a 5v DC wallwart, just make sure it's not one of those 500ma output wienies. You'll be back in the same boat.
You'll want to get an external harddrive supply or something similar that can output 2-3amps (2000-3000 ma) of current.
(EDIT) Connecting to the PC Supply with a fused line is also good as in the link that Hoopz points to.