3G Memory sees only 1G Memory

Originally installed two sticks of 1/2 Gig sticks then I filled the three memory slots with 1 Gig sticks of memory. During the booting process one Gig is seen then when I changed to three Gigs of memory only one Gig is recognized. How do I get the computer to recognize the additional memory?

what is your mobo ?

Edit: & how old is it ?

My motherboard is IWILL XP333-R 32404A October 15 2001
My Processor is AMD AthlonXP 1331+ 2001
My BIOS is Award Modular V6.00PG 2001