what is your psu? i had the same issue 2 time and i had different solutions ,i plugged this
motherfuckers wrong in the 1th time
in the 2th time my cpu fan didnt work in the cpu fan connector on the mobo but it worked in system fan
all of this made my pc turn the fans and everything for 3 seconds and then turn off.
it also can be
faulty ram(mabye one of the ram sockets doesn't work)
faulty bios
faulty mobo
and more,i would recommend watching this video:
Do you still need help @Wave ?