Is it my motherboard, CPU, or RAM?

My computer is having some issues. The computer will turn on, all the fans, lights, drives, and my graphics card will turn on, but the computer won't display anything on the screen. It won't show my BIOS, it wont show anything it says "NO SIGNAL" on the screen. I tested the monitor (working), used different cables (all working), tested the Power Supply (working), the graphics card (working), the CD drive (working) and the Hard Drive (working). The only parts I can't test are the ones I can't test by themselves or in my other computer, which are my Motherboard, CPU, and RAM. I need to know which one it is so I can get a replacement as they are all under warranty.