I will say that the high power LED's that are sold with the current limiting already on the heatsink are awfully nice to use, even if they are a bit more expensive. I've paid as much as two dollars a watt, but usually they end up being around half that, at a buck a watt, give or take, for whites in the 2watt range. Feed them anything from 3.2 to 6v, they self limit at 500ma or thereabout. Here's a link:
EDIT: Got diverted while getting the link, hehehe:
Now, I will say that it does require extra heat sinking in actual operation- but running a couple in series from 12v works great. I suppose running three ought to be fine at 12v. Its a lot of light for cheap money... The link has them at under a buck each for ten. I am not connected to it in any way other than saying I've bought them and though they seem a little dimmer than expected given the power draw, but not so much that you'd actually care much... I use an NPN transistor to switch the ground side to provide PWM.
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They are awfully convenient to use, when you don't want to diddle around with constant current sources...