I think David, that that is one of the motherboards that is supported for upto 2gb of RAM? If so then it wont register the rest, although I am not entirely sure. I had a similar problem but was able to download a bios fix, that cracked my motherboard to allow more, but I ended up with more problems, so reverted back :o)
One question? Why on earth do you need or want 3gb of RAM in a home machine? LOL