The ram is not seated fully, in the wrong slots or not plugged in.
If reseating the RAM fails then we have to think the machine now needs repairs.
I recently tried to install new memory on my desktop. I wanted to put in 4g of ram, but found out my gateway system only supports 2g of ram. After I tried to start my system with the new memory, it didn't boot up...just beeped three times. I took out the new memory and reinstalled the old memory and the same thing happened...just three beeps and a black screen. Someone suggested replacing my BIOS battery which I did and still nothing....any ideas what to do next.