3fo usually means boot disk not found.
bad sectors in boot section of hard drive
bad connection between drive and motherboard (sata cable)
SATA controller failed on motherboard BIOS
the hard drive shows up in the bios and shows up on a external caddy which probably means its not 1 or 3
Can you get to adavanced tools menu by tapping F* as it boots?
if you can get to command prompt type
when its checked the drive it may then boot OK if not go back to Advance tools and run Start up repair - if it says can fault run the repair at least 5 times.