Memory Upgrade problem

I recently upgraded my refurbed Dell Pre T3500 with 12 G memory. 3 sticks of 4g. I placed them in dims 1,2,3 and though my computer works ok every time I start it I get, Alert! Non-optimal memory population detected. populate Dimms as specified in the service manual. Strike F1 key to continue. F2 to run the setup utility. Press F5 to run onboard diagnostics. Any ideas what's happening? I ran the diagnostics. When I hit F1 it works good and somewhat normal?

Thanks, Greg