RE : My Computer Turns On For 3 Seconds, Then Shuts Off !!
This may help someone reading this thread.
I had this problem in the last few days after a newly built high end graphics computer, and on first power-on it shuts down 3 seconds later.
Double checked all connections without sucess.
After reading this thread, I worked my way changing PSU, heatsink, PCI cards, HD and the CPU - NO LUCK it still shuts down.
I have noticed that after it shuts down - and with power off from the mains plug - but as I plug it back to power from the mains, it powred on its own and shuts down again.
This, in my case, lead me to the CULPRIT, the new tower case front
ON-OFF button (back spring) was sticking on - ON - !!
So its worth checking it first, even if its a new case.