Dimm slot 3 is shared between itself and number two. This is why you cannot populate all three slots unless you have single sided memory.
With three 512MB DIMM modules, maximum memory is 1024MB. 512MB SS modules are more expensive and rather uncommon. They use the same chips as 1024 modules, they only have the chips on one side. If you want to use all of that RAM, you'll need a SiS based board or upgrade to GB in a few weeks and grab another 512 module and enjoy 2048MB and more speed! 🙂
Cheers!