The KA378R05 is a low-dropout voltage regulator from Fairchild Semiconductors. From the page, "Dropout voltage of KA378R05 is below 0.5V in full rated current(3A)". It does have a 4-pin layout, the fourth pin is "Disable". Just tie that pin to your supply line and it'll work fine.
Available at Mouser for $1.58, probably elsewhere too. I got mine as free samples from Fairchild, so you may find some that way too.
2009 July 01, added quickie schematic