Hello Jackfrost,
Although your mobo have 3 memory slots, it will only support and recognize dual-sided modules in any 2 of the three - that is why a lot of these 845 boards only sport 2 slots.... If you want to populate all 3, you will have to use 2 single and 1 double or 3 single sided modules.
Hope this helps a bit!
Regards,
MrBean
South Africa