You'll notice that the slots are usually coloured. 2 of one colour (usually 1&2) the other different (3).
You can get the DDR buy putting matching ram (2x256 for example) in 1&3 or 2&3
However, you could take the same 2x256 and put them in 1&2 and then add a matching 512 and put it in 3. They will then be used as 2x512.
When I say matching, I mean all the same type (PC 2700 for example), and I would go so far as to make sure they were the same brand too.