So I built my first full build about a month ago, everything is perfect and running great except one thing. When I power my PC on, the system starts up as normal and everything is fine until it powers itself back off for about 3 seconds and then turns itself back on then goes on as normal and starts up as if nothing happened. This happens every single time I turn my PC on from being completely shut down. I've actually seen it one time go On>Off>On>Off>On before. No error messages pop up anywhere. I can go into my components if it helps, although I hope it's an easy fix.
 It powers itself down 3 seconds after pushing my power button
From your description I'm assuming you have a Sandy Bridge system.
This is a common problem, and the only consistent fix is a BIOS update for the mainboard. Check to see whether you have the newest BIOS version -- if you do, then you'll have to wait for a newer one. If you don't have the newest one, then update it and see if that fixes the problem.