You can't run 3 slots in dual channel. If the motherboard is happy with the memory, it'll run all three modules in single channel mode.
That's not necessarily true of older motherboards based on the nForce2 chipset, in which case it's not about the number of sticks, but how the memory banks are separated.
For example, if slots 1 and 2 are part of memory bank A and slot 3 is memory bank B, you could install 512MB sticks in slots 1 and 2 and a 1GB stick in slot 3, effectively filling each memory bank (not slot) with 1GB of RAM, for 2GB total in dual-channel mode.
There used to be an Abit FAQ page that confirmed the AN7 cooperated this way. It read:
"You will see the string 'Dual Channel enabled' on the POST screen while RAM modules installed on DIMM 2&3, DIMM 1&3 or all three DIMM slots. If you installed only one memory module on either one slot or two memory modules on DIMM 1&2, you will see the string 'Single Channel enabled' on the POST screen. Please refer to the following ABIT website for the memory modules we have tested on AN7."
The page is long dead now, but used to appear here: http://fae.abit.com.tw/eng/faq/qa/2003/2003112105.htm